cases aid in respiration to a certain extent, yet that extent is
so small as to be left out of consideration altogether. He regards
the cerata in both the two classes mentioned above as “of primary
importance in giving to the animals, by their varied shapes and
colours, appearances which are in some cases protective, and in others
conspicuous and warning.”
Thus, for instance, _Tritonia plebeia_, which is fairly abundant at
Puffin and Hilbre Is., appears always to be found creeping on the
colonies of a particular polyp, _Alcyonium digitatum_, and nowhere
else. The specimens in each colony of the polyp differ noticeably
both in the matter of colour, and of size, and of varied degrees of
expansion. The _Tritonia_ differs also, being marked in varied tints
of yellow, brown, blue, gray, black, and opaque white, in such a way
as to harmonise with the varied colours of the _Alcyonium_ upon which
it lives. The cerata on the back of the _Tritonia_ contribute to this
general resemblance. They are placed just at the right distance apart,
and are just the right size and colour, to resemble the crown of
tentacles on the half-expanded polyp.
Similarly, _Doto coronata_, which, when examined by itself, is a very
conspicuous animal, with showy, bright-coloured cerata, is found by
Professor Herdman to haunt no other situations but the under side of
stones and overhanging ledges of rock which are colonised by a hydroid,
known as _Clava multicornis_. The _Doto_ is masked by the tentacles and
clusters of sporosacs on the zoophyte, with whose colouring and size
its own cerata singularly correspond. A similar and even more deceptive
correspondence with environment was noticed in the case of the very
conspicuous _Dendronotus arborescens_.
In these cases, the colouring and general shape of the cerata are
protective, _i.e._ they match their surroundings in such a way as to
enable the animal, in all probability, to escape the observation of
its enemies. According to Professor Herdman, however, the brilliant
and showy coloration of the cerata of _Aeolis_ is not protective
but ‘warning.’ _Aeolis_ does not hide itself away as if shunning
observation, like _Doto_, _Tritonia_, and _Dendronotus_; on the
contrary, it seems perfectly fearless and indifferent to being noticed.
Its cerata are provided with sting-cells, like those of Coelenterata,
at their tips, and its very conspicuousness is a warning to its
enemies that they had better not try to attack it, just as the showy
white tail of the skunk acts as a sort of danger-signal to its own
particular foes. It is important for the _Aeolis_, not merely to _be_
an unpalatable nettle in animal shape, but also to be conspicuous
enough to prevent its being experimented upon as an article of food, in
mistake for something less nasty.
Professor Herdman subsequently conducted some experiments[158] with
fishes, with the view of testing his theory that the shapes and colours
of Nudibranchs serve the purpose either of protection or warning, and
bear direct relation to the creature’s edibility. These experiments,
on the whole, distinctly tended to confirm the theory. _Aeolis_ was
evidently very nasty, and probably stung the mouths of the fishes who
tried it. For the complete success of the theory, they ought to have
let it severely alone, but the fish were evidently accustomed to make a
dash at anything that was dropped into their tank. Another conspicuous
mollusc, _Ancula cristata_, was introduced, Professor Herdman and his
collaborator each commencing operations by eating a live specimen
themselves. They found the taste pleasant, distinctly like that of an
oyster. The fish, however, when the experiments were conducted under
conditions which made the scene as much like ‘real life’ as possible,
did not agree with Professor Herdman. The _Ancula_ crawled over various
parts of the tank for several days untouched by the fish, who sometimes
went close to them and looked at them, but never attempted to taste
them. Experiments with species whose colours were protective, such as
_Dendronotus_, were also conducted, and the decided edibility of these
species was established, the fish competing eagerly for them, and
tearing them rapidly to pieces.
Mr. W. Garstang, of the Plymouth Laboratory of the Marine Biological
Association, confirms[159] Professor Herdman’s views as to the shape
and colour of Opisthobranchs. _Pleurobranchus membranaceus_ is known
to secrete, on the surface of the body, an acid which reddens blue
litmus paper. It is, therefore, no doubt distasteful to fish, which
all abominate the taste of acids, and is conspicuously marked with
red-brown and yellowish ‘warning’ colours. _Haminea_ and _Philine_, on
the other hand, are good to eat, and consequently possess ‘protective’
coloration. _Runcina Hancocki_, which is of a brown colour, crawls
over brown mud and weeds, but avoids green weeds, on whose surface it
would appear conspicuous. _Elysia viridis_ varies its colour according
to its habitat, being green when on green weeds, and dark olive,
brown, or reddish brown, on pools among tufts of littoral algae. Green
specimens of _Hermaea dendritica_ were kept in captivity, and placed in
a dish with green and red sea-weeds. They were never observed crawling
upon the red weed, upon which they would have been very conspicuous.
_Archidoris flammea_ occurred on bright red sponges, to which its
colour was so closely assimilated that Mr. Garstang at first quite
overlooked it. _Goniodoris castanea_ was found under stones, feeding on
compound Ascidians (_Botryllus_), which it sufficiently resembled to be
very inconspicuous in that position.
Again, _Jorunna Johnstoni_ lives[160] upon stones on our southern
coast, associated with a certain sponge (_Halichondria_ sp.), which it
resembles so closely in outline, in colour, in character of surface,
and in its projecting plumes, as to make it very difficult even for
the careful observer to distinguish the one from the other. And, since
fishes, are known to be distinctly averse to sponges of any kind as an
article of food, this resemblance must be decidedly to the advantage
of the _Jorunna_. Another Nudibranch (_Calma glaucoides_ A. and H.)
imitates the ova of certain fishes, on which it feeds. Its elongated
and depressed form of body, transparent integuments, and silvery gray
papillae combine to give it a strong resemblance to the spawn of the
fish, which is deposited on stones, the roots of _Laminaria_, etc.[161]
The common _Lamellaria perspicua_ appears to possess the power of
protectively assimilating its colour, markings, etc., to the Ascidians
on which it lives. A recent case, occurring off the Isle of Man, is
thus described by Professor Herdman.[162] “The mollusc was on a colony
of _Leptoclinum maculatum_, in which it had eaten a large hole. It lay
in this cavity so as to be flush with the general surface; and its
dorsal integument was not only whitish with small darker marks which
exactly reproduced the appearance of the _Leptoclinum_ surface with
the ascidiozooids scattered over it, but there were also two larger
elliptical clear marks which looked like the large common cloacal
apertures of the Ascidian colony.... Presumably the _Lamellaria_
escapes the observation of its enemies through being mistaken for part
of the _Leptoclinum_ colony; and the _Leptoclinum_, being crowded like
a sponge with minute sharp-pointed spicules, is, I suppose, avoided as
inedible (if not actually noxious through some peculiar smell or taste)
by carnivorous animals which might devour such things as the soft
unprotected mollusc.”
=Parasitic Mollusca=
Various grades of parasitism occur among the Mollusca, from the true
parasite, living and nourishing itself on the tissues and secretions of
its host, to simple cases of commensalism. Some authors have divided
these forms into endo- and ectoparasites, according as they live
inside or outside of their host. Such a division, however, cannot
be rigidly carried out, for certain forms are indifferently endo-
and ecto-parasitical, while others are ecto-parasitic in the young
form, and become endo-parasitic in the adult. It will be convenient,
therefore, simply to group the different forms according to the home on
which they find a lodgment.
[Illustration: FIG. 29.--_Magilus antiquus_ L.: =A=, the adult,
imbedded in coral, which has been broken away to show the tube;
=B=, the young (free) form.]
On _Sponges_.--_Vulsella_ and _Crenatula_ almost invariably occur
in large masses of irregular shape, boring into sponges. They are
especially abundant on Porifera from the Red Sea. Corals form a
favourite home of many species, amongst which are several forms
of _Coralliophila_, _Rhizochilus_, _Leptoconchus_, and _Sistrum_.
_Rhizochilus_ is a very singular creature, inhabiting branching corals.
When adult, it forms irregular shelly extensions of both the inner and
outer lips, which adhere to the shafts of the coral, or to the surface
of neighbouring shells; at length the aperture becomes completely
closed with the exception of the siphonal tube, which becomes long, and
consists of the same shelly material. The common _Magilus_ (Fig. 29),
from the Red Sea and Indian Ocean, in the young form is shaped like
a small _Buccinum_. As the coral (_Meandrina_) to which it attaches
itself grows, the _Magilus_ develops at the mouth a long calcareous
tube, the aperture of which keeps pace with the growth of the coral,
and prevents the mollusc from being entombed. The animal lives at the
free, or outer, end of the tube, and is thus continually shifting its
position, while the space it abandons becomes completely closed by a
mass of solid calcareous matter. Certain species of _Ovula_ inhabit
Gorgonia, assuming the colour, yellow or red, of their host, and, in
certain cases, developing, probably for prehensile purposes, a pointed
extension of the two extremities of the shell. _Pedicularia_, a form
akin to _Cypraea_, but with a more patulous mouth, inhabits the common
_Corallium rubrum_ of the Mediterranean, and another species has been
noticed by Graeffe[163] on _Melithaea ochracea_ in Fiji.
On _Echinodermata_.--(_a_) _Crinoidea._ _Stylina comatulicola_ lives on
_Comatula mediterranea_, fixed to the outer skin, which it penetrates
by a very long proboscis; the shell is quite transparent.[164] A
curious case of a fossil parasite has been noticed by Roberts.[165]
A _Calyptrea_-shaped shell named _Platyceras_ always occurred on the
ventral side of a crinoid, encompassed by the arms. For some time this
was thought to afford conclusive proof of the rapacity and carnivorous
habits of the echinoderm, which had died in the act of seizing its
prey. Subsequent investigations, however, showed that in all the
cases noticed (about 150) the _Platyceras_ covered the anal opening
of the crinoid in such a way that the mouth of the mollusc must have
been directly over the orifice of the anus. (_b_) _Asteroidea._ The
comparatively soft texture of the skin of the starfishes renders
them a favourite home of various parasites. The brothers Sarasin
noticed[166] a species of _Stilifer_ encysted on the rays of _Linckia
multiformis_. Each shell was enveloped up to the apex, which just
projected from a hole at the top of the cyst. The proboscis was long,
and at its base was a kind of false mantle, which appeared to possess
a pumping action. On the under side of the rays of the same starfish
occurred a capuliform mollusc (_Thyca ectoconcha_), furnished with a
muscular plate, whose cuticular surface was indented in such a way
as to grip the skin of the _Linckia_. This plate was furnished with
a hole, through which the pharynx projected into the texture of the
starfish, acting as a proboscis and apparently furnished with a kind
of pumping or sucking action. Adams and Reeve[167] describe _Pileopsis
astericola_ as living ‘on the tubercle of a starfish,’ and _Stilifer
astericola_, from the coast of Borneo, as ‘living in the body of a
starfish.’ In the British Museum there is a specimen of _Pileopsis
crystallina_ ‘in situ’ on the ray of a starfish, (_c_) On the brittle
starfishes (_Ophiuroidea_) occur several species of _Stiliferina_.
(_d_) _Echinoidea._ Various species of _Stilifer_ occur on the ventral
spines of echinoids, where they probably subsist on the excreta, and
are sometimes found imbedded in the spines themselves. _St. Turtoni_
occurs on the British coasts on several species of _Echinus_, and
_Montacuta substriata_ frequents _Spatangus purpureus_ and certain
species of _Echinocardium_, _Cidaris_, and _Brissus_. _Lepton
parasiticum_ has been described from Kerguelen I. on a _Hemiaster_,
and a new genus, _Robillardia_, has recently been established[168]
for a _Hyalinia_-shaped shell, parasitic on an _Echinus_ from
Mauritius. (_e_) _Holothurioidea._ The ‘sea-cucumbers’ afford lodgment
to a variety of curious forms, some of which have experienced such
modifications that their generic position is by no means established.
_Entoconcha_ occurs fixed by its buccal end to the blood-vessels of
certain _Synapta_ in the Mediterranean and the Philippines. _Entocolax_
has been dredged from 180 fath. in Behring’s Straits, attached by its
head to certain anterior muscles of a _Myriotrochus_.[169] A curious
case of parasitism is described by Voeltzkow[170] as occurring on
a _Synapta_ found between tide-marks on the I. of Zanzibar. In the
oesophagus of the _Synapta_ was found a small bivalve (_Entovalva_),
the animal of which was very large for its shell, and almost entirely
enveloped the valves by its mantle. As many as five specimens occurred
on a single _Synapta_. In the gut of the same Holothurian lived a small
univalve, not creeping freely, but fixed to a portion of the stomach
wall by a very long proboscis which pierced through it into the body
cavity. This proboscis was nearly three times as long as the animal,
and the forward portion of it was set with sharp thorns, no doubt in
order to enable it to retain its hold and resist evacuation. Various
species of _Eulima_ have been noticed in every part of the world, from
Norway to the Philippines, both inside and outside Holothurians.[171]
_Stilifer_ also occurs on this section of Echinoderms.[172]
On _Annelida_.--_Cochliolepis parasiticus_ has been noticed under
the scales of _Acoetes lupina_ (a kind of ‘sea-mouse’) in Charleston
Harbour.[173]
On _Crustacea_.--A mussel, ⅜ in. long, has been found[174] living
under the carapace of the common shore-crab (_Carcinus maenas_), and
one case has been noticed[175] where two mussels, one of several
months’ growth, the other smaller, well secured by their byssi, were
found under the abdomen of the same species, in such a position as to
force the appendages apart and askew. These, however, are not so much
cases of parasitism as of involuntary habitat, the mussel no doubt
having become involved in the branchiae and the abdomen of the crab in
the larval form.
On _Mollusca_.--A species of _Odostomia_ (_pallida_ Mont.) is found on
our own coasts on the ‘ears’ of _Pecten maximus_, and also[176] on the
operculum of _Turritella communis_. Another species (_O. rissoides_)
frequently occurs in hiding under beds of mussels, but it is not clear
whether the habitat is due to parasitism, or simply to the fact that
the mass of mussels, knitted together and to the rock by the byssi,
affords the _Odostomia_ a safe lurking-place. At Panama the present
writer found _Crepidula_ (2 sp.) plentiful on the opercula of the
great _Strombus galea_ and of _Cerithium irroratum_. In each case the
parasite exactly fitted the size of the operculum, and had assumed its
colour, dark brown or chestnut. _Amalthea_ is very commonly found on
_Conus_, _Turbo_, and other large shells from the South Pacific, but
this is probably not a case of parasitism, but simply of convenience of
habitat, just as young oysters are frequently seen on the carapace and
even on the legs of large crabs.
[Illustration: FIG. 30.--_Crepidula onyx_ Sowb., parasitic on the
operculum of _Strombus galeatus_ Swains., Panama.]
On _Tunicata_.--_Lamellaria_ deposits its eggs and lives on an Ascidian
(_Leptoclinum_), and the common _Modiolaria marmorata_ lives in
colonies imbedded in the test of _Ascidia mentula_ and other simple
Ascidians.
[Illustration: FIG. 31.--Two species of _Eulima_: =A= is sessile
on the skin of a Holothurian, through which it plunges its
sucking proboscis (_Pr_); =B= creeps freely in the stomach of a
Holothurian. (After K. Semper.)]
Special points of interest with regard to parasitic Mollusca relate
to (1) _Colour_. This is in most cases absent, the shell being of
a uniform hyaline or milky white. This may be due, in the case of
the endo-parasitic forms, to absence of light, and possibly, in
those living outside their host, to some deficiency in the nutritive
material. A colourless shell is not necessarily protective, for
though a transparent shell might evade detection, a milk-white hue
would probably be conspicuous. (2) _Modifications of structure._
These are in many cases considerable. _Entoconcha_ and _Entocolax_
have no respiratory or circulatory organs, and no known nervous
system; _Thyca_ and certain _Stilifer_ possess a curious suctorial
apparatus; the foot in many cases has aborted, since the necessity for
locomotion is reduced to a minimum, and its place is supplied by an
enormous development of the proboscis, which enables the creature to
provide itself with nutriment without shifting its position. K. Semper
notices a case where a _Eulima_, whose habitat is the stomach of a
Holothurian, retains the foot unmodified, while a species occurring
on the outer skin, but provided with a long proboscis, has lost its
foot altogether.[177] Special provision for holding on is noticed in
certain cases, reminding us of similar provision in human parasites.
Eyes are frequently, but not always wanting, even in endo-parasitic
forms. A specially interesting modification of structure occurs in
(3) the _Radula_ or ribbon-shaped arrangement of the teeth. In most
cases of parasitism (_Eulima_, _Stilifer_, _Odostomia_, _Entoconcha_,
_Entocolax_, _Magilus_, _Coralliophila_, _Leptoconcha_) it is absent
altogether. In _Ovula_ and _Pedicularia_, genera which are in all
other respects closely allied to _Cypraea_, the radula exhibits marked
differences from the typical radula of the Cypraeidae. The formula
(3·1·3) remains the same, but the laterals are greatly produced
and become fimbriated, sometimes at the extremity only, sometimes
along the whole length. A very similar modification occurs in the
radula of _Sistrum spectrum_ Reeve, a species which is known to live
parasitically on one of the branching corals. Here the laterals differ
from those of the typical _Purpuridae_ in being very long and curved at
the extremity. The general effect of these modifications appears to be
the production of a radula rather of the type of the vegetable-feeding
_Trochidae_, which may perhaps be regarded as a link in the chain of
gradually-degraded forms which eventually terminate in the absence of
the organ altogether. The softer the food, the less necessity there
is for strong teeth to tear it; the teeth either become smaller and
more numerous, or else longer and more slender, and eventually pass
away altogether. It is curious, however, that the same modified form
of radula should appear in species of _Ovula_ (_e.g._ _ovum_) and that
the same absence of radula should occur in species of _Eulima_ (_e.g._
_polita_) known to be not parasitic. This fact perhaps points back to
a time when the ancestral forms of each group are parasitic and whose
radulae were modified or wanting, the modification or absence of that
organ being continued in some of their non-parasitical descendants.
=Commensalism=
Mollusca are concerned in several interesting cases of commensalism,
or the habitual association of two organisms, as distinguished from
parasitism, where one form preys more or less upon the other.
Mr. J. T. Marshall has given[178] an interesting account of the
association of _Montacuta ferruginosa_ with _Echinocardium cordatum_.
The Echinoderm lives in muddy sand in Torbay, at a depth of about 6
inches, and the _Montacuta_ lives in a burrow leading from its ventral
end and running irregularly in a sloping direction for 3 or 4 inches,
the burrow, which is made by a current from the Echinoderm, being
almost exactly the width of the _Montacuta_. The _Montacuta_ were
always arranged in the burrows in order of size, the largest being
close to the Echinoderm, and the smallest of a string of about six at
the other end of the burrow. In another part of S. Devon, where the
sand was soft and sloppy, the Echinocardia rise to the surface and
travel along the sand; in this case the _Montacuta_ were attached to
their host by means of a byssus, and were dragged along as it travelled.
The Rev. Dr. Norman has noted[179] a somewhat similar habitat for
_Lepton squamosum_. This rare little British species was found at
Salcombe, living in the burrows of _Gebia stellata_, in all probability
feeding upon the secretions from the body of the crustacean. Dr.
Norman suggests that the extreme flatness of the shell of the _Lepton_
is of great advantage in enabling it not to get in the way of the
_Gebia_ as he scuttles up and down his burrow. Another species of
_Lepton_ is found on the coast of Florida in a precisely similar
locality,[180] while a third species, occurring on the Oregon and
California coasts, actually attaches itself to the inner surface of the
abdomen of a _Gebia_.[181]
[Illustration: FIG. 32.--_Ephippodonta Macdougalli_ Tate, S.
Australia. =A=, Burrow of prawn, the X indicating the position of
the mollusc; _sp_, sponge. =B=, Ventral view of _Ephippodonta_;
_by_, byssus; _f_, foot; _m_, mantle; _mm_, fused mantle borders.
=C=, View of interior of shells; _h_, hinge; m´m´, adductor
muscles. (=A= × ½; =B= and =C= × 2.)]
A very singular case of commensalism has been recently discovered
with regard to a genus of Australian bivalve shells, _Ephippodonta_.
This genus is never found except in the burrow of a species of prawn
(_Axius plectorhynchus_ Str.). For some reason at present unexplained,
the burrow of this particular prawn appears to be exceedingly popular
as a habitat for certain bivalves, for, besides two species of
_Ephippodonta_, a _Kellia_ and three _Mylitta_ are found there, and
there alone. Sometimes the prawn, when the rock is hard, builds a
tunnel of mud upon it, at other times it excavates the soft calciferous
sandstone. “This burrow is lined with a tenacious brown mud, composed
of excrementitious matter; and, in addition to the mud lining, there
is always more or less present an orange-coloured sponge which I have
never found elsewhere. Upon the mud or sponge, and adhering very
closely, are found the _Ephippodonta_. They quickly form a pit-like
depression by means of their foot, and appear almost covered by the
mud.” During the winter months (March-July) the prawn appears to fill
his burrow, possibly as a provision against stormy weather, with large
quantities of minced seaweed, underneath which immense numbers of
very young _Ephippodonta_ are found living.[182] The extreme flatness
of the _Ephippodonta_ must be due to the same cause as the flatness
of the _Lepton_ noticed above, namely, the necessity of not impeding
or interfering with the lively motions of the prawn. In the case of
_Lepton_ the two valves close completely and the shell is still very
flat; in _Ephippodonta_, on the other hand, the same result is produced
by the valves being opened to their widest possible extent. As in
_Entovalva_, a continuation of the mantle covers the outer surface of
the shell.
=Variation=
It is a familiar experience to the student, not only of the Mollusca,
but of every branch of animal or vegetable life, to come across
examples which exhibit certain slight deviations from the type
form as usually understood. These deviations may be more or less
pronounced, but, as a rule, a series of forms can be discovered,
gradually leading up to or down from the type. The definition of what
constitutes a species,--and, still more, the rigid application of
such definition--will always remain a difficult task, so long as the
personal element persists in him who defines.[183] What seems to one
authority ample ground for distinction of species, another may regard
as of comparatively trivial importance. The practical outcome of these
divergent views is sufficiently illustrated by the attitude of Mr. F.
P. Marrat on the one hand, and of what may be called the modern French
school of conchologists on the other. Mr. Marrat holds, or held, that
the great genus _Nassa_, of which more than 150 species are generally
recognised, is one shell (species) in an endless variety of forms. The
modern French school go to the other extreme, and apparently proceed
upon the view that almost any difference in form, however slight, is
sufficient to constitute a separate species.
It will be generally admitted, however, that some _structural
difference_ in the organisation of the animal (as distinct from that
of the shell alone) is necessary for the permanent constitution of
specific rank.[184] What _amount_ of structural difference is required,
what particular organ or organs must exhibit this difference, will
depend largely upon the idiosyncrasy of the observer. But if this, or
something like this definition of a species be accepted, it will follow
that a so-called ‘variety’ will be a form which exhibits differences
from the type which do not amount to permanent structural differences
in the organisation of the animal. The final court of appeal as to what
affords sufficient evidence for ‘permanent structural differences’ will
have to be, as with Aristotle of old, the judgment of the educated man.
It is, however, more to our present purpose to discuss the _causes of
variation_ than to lay down definitions of what variation is. One of
the most obvious causes of variation lies in a change or changes in
the environment. If we may assume, for the moment, that the type form
of a species is the form which is the mean of all the extremes, and
that this form is the resultant of all the varied forces brought to
bear upon it, whether of food, climate, temperature, competition of
numbers, soil, light, amount of water, etc., it will follow that any
change in one or more of these forces, if continuous and considerable,
any change, in other words, of the environment, will produce its
effect upon the organism in question. And this effect will be for
the better or for the worse, according to the particular nature of
the change itself as tending towards, or away from, the _optimum_ of
environment for the species concerned. Hence may be produced varieties,
more or less marked according to the gravity of the change, although
it must be noted that at times a change apparently unimportant from
our point of view, will produce very marked results upon the species.
It is indeed scarcely possible to predict with any certainty, in the
present state of our knowledge (beyond certain broad results) what will
be the particular effect upon a species of any given change in its
surroundings.
=Effects of Change in the Environment as tending to produce Variation.=
(_a_) _Changes in Climate, Temperature, Elevation, etc._--In the
eastern basin of the Baltic the marine Mollusca are much more stunted
than in the western.[185] For instance, _Mytilus edulis_ near Kiel
is 8–9 cm. long, while near Gothland it only attains a length of
3–4 cm. Mollusca living at only a shallow depth (_e.g._ _Tellina
balthica_, _Mya arenaria_, _Cardium edule_) do not differ much in
size in different parts of the Baltic, but in the far eastern basin
the calcareous layers of the shells of _Mya arenaria_ and _Tellina
balthica_ are extraordinarily thin, and disappear very rapidly
after death, leaving only the cuticular membrane, still united by
the ligament, in a perfect state of preservation. These remarkable
variations are no doubt to a large extent due to the violent changes
of temperature which are experienced in the Baltic, and by which the
steady development of the animals in question is interrupted and
thrown out of gear. The same species occur on the coasts of Greenland
and Iceland, where they attain a considerably larger size than in the
Baltic, in spite of the lower mean temperature, probably because their
development is not interrupted by any sudden change from cold to heat
or _vice versâ_.
Karl Semper has shown that _Limnaea stagnalis_ is developed, lives
and feeds best in a mean temperature of about 20° C. (= 68° F.). This
mean, however, must not be the mean of two distant extremes, for the
_Limnaea_ cannot digest its food and grow in a temperature which is
less than 14° or 15° C. (= 57° or 59° F.), or more than 30° to 32° C.
(= 86° to 90° F.). In certain localities, therefore, the interruption
to the growth of this species must be serious and prolonged, and may
tend towards the production of more or less dwarfed varieties. Thus
specimens from Malham Tarn, a lake in Yorkshire 1250 feet above the
sea, are permanently dwarfed, and have a very thin and fragile shell.
_Limnaea peregra_ in the Pyrenees, Alps, and Himalayas is generally of
a very delicate form and dwarfed habit, while the small variety known
as _lacustris_ occurs, according to Jeffreys, only in mountain lakes in
Zetland, Scotland, Ireland, and N. England. Specimens brought by Mr.
Bateson from lakes near the Sea of Aral, which are salt for some months
and comparatively fresh for others, exhibit clearly the effect of
changes in the environment (Figs. 33 and 34). Excess of heat produces
similar results to excess of cold. _L. peregra_ var. _thermalis_,
found in the warm springs of the Pyrenees and the Vosges, and the var.
_geisericola_, from the hot water of the Iceland geysers, are alike
thin and dwarfed forms.
Many instances may be given of ‘varieties due to locality.’ In some of
these, the cause which predisposes towards variation can be inferred
with some approach to certainty, in others we must be content to note
the fact, without at present being able to perceive its explanation.
[Illustration: FIG. 33.--Four examples of _Limnaea peregra_
Müll., from salt marshes near the Sea of Aral, showing different
effects produced by abnormal conditions of life.]
[Illustration: FIG. 34.--Four examples of _Limnaea stagnalis_
L., from marshes in the Aral district which are salt for several
months in the year, illustrating variation produced by changes in
the environment. × ½.]
Desert specimens of widely distributed species, _e.g._ _Helix pomatia_,
_H. niciensis_, _H. pisana_, _Leucochroa candidissima_ are much thicker
than the type, and tend to lose all trace of coloured bands. These
modifications are clearly the means of preventing evaporation of
moisture, the dull white or grayish brown colour being calculated to
absorb the smallest possible amount of heat. Desert shells in all parts
of the world (_e.g._ N. Africa, Arabia, Central Asia, S. Africa, W.
America) have been noticed to exhibit these peculiarities.
A very singular case of the reverse process, _i.e._ the production
of darkened forms of shell through cold, has been noticed by Fischer
as characteristic of the marine shells of the west coast of South
America.[186] This melanism is especially noticeable in _Trochus_,
_Turbo_, _Chiton_, _Mitra_, and _Pleurotoma_, and is attested by the
specific names, not merely expressive of actual blackness (_e.g._
_nigerrimus_, _ater_, _atramentarius_, _maurus_), but also of a
generally lugubrious tone (_e.g._ _moestus_, _funebralis_, _tristis_,
_lugubris_, _luctuosus_). It is highly probable that this concurrence
of specific melanism (which stands quite alone in the world) is due
to the cold polar current which impinges on the Chilian coasts, for
the same genera occur on the opposite shores of the continent without
exhibiting any trace whatever of this mournful characteristic.
It is a well-known fact, attested by many observers, that our common
_Limax agrestis_ as well as the young of _Arion ater_ become decidedly
darker in summer than in winter. If these slugs were accustomed to
disport themselves in the sun, it might have been suggested that this
increased darkness of colour tended to absorb more of the heat rays.
But since this is not the case, the result is probably due to some
unexplained effect of higher temperature. According to Lessona and
Pollonera, the length of the keel in _Limax arborum_ varies greatly in
different parts of Italy, being shorter in specimens from low ground,
but much longer in those inhabiting more elevated regions. The longer
the keel, the more obscure the colouring becomes, so that in the
Upper Alps of Piedmont individuals are practically black. Roebuck has
observed that Scottish specimens of this same slug are much darker
and less translucent than English forms. According to Simroth, our
common black slug, _Arion ater_, is a northern type, which in more
southern latitudes assumes the form known as _A. rufus_. Similarly
_Limax maximus_ “in its northern form _cinereo-niger_ is almost wholly
black, but in the more genial climate of Italy develops a series of
brilliantly coloured and strikingly marked variations which have
received numerous distinctive names from Italian limacologists.”[187]
According to Scharff, however[188] (who regards the colours of slugs as
in the main protective), these dark forms are by no means exclusively
northern, being found equally on the parched plains of Spain and
Portugal, and in the bleak climate of Norway. The same authority
observes that similar forms occur both in the dry regions of E.
Germany, and in the very humid district of western Ireland.
It appears unquestionable that marine genera from high northern
latitudes are provided with shells of uniform colour, or whitish with a
pale brown epidermis; spots, bands, or stripes seldom occur. The arctic
forms of _Buccinum_, _Trophon_, _Chrysodomus_, _Margarita_, _Crenella_,
_Leda_, _Yoldia_, _Astarte_ illustrate this fact. In the more temperate
seas of Europe, colours tend on the whole to increase, although there
are certain genera (_e.g._ _Pecten_) which are not more brightly
coloured in Mediterranean than in Icelandic waters.
Land Mollusca inhabiting the mainland of a continent not unfrequently
become smaller when they have spread to adjacent islands where perhaps
the rainfall is less abundant or the soil and food-supply less nicely
adjusted to their wants. _Orthalicus undatus_ is decidedly larger on
the mainland of S. America than on the adjacent islands of Trinidad and
Grenada. Specimens of _Bulimulus exilis_ from Barbados are invariably
broader and more obese than those from S. Thomas, while those from the
volcanic island of S. Lucia, where lime is deficient, are small and
very slender. _Streptaxis deformis_, as occurring at Trinidad, is only
half the size of specimens from Georgetown, Demerara.[189]
Certain localities appear, for some unexplained reason, to be
particularly favourable to the production of albino varieties. The
neighbourhood of Lewes, in Sussex, has produced no fewer than fourteen
of these forms of land Mollusca and five of fresh-water.[190]
Our common _Helix aspersa_, as found near Bristol, is said to be ‘dark
coloured’; about Western-super-mare ‘brown, with black markings’;
near Bath ‘very pale and much mottled’; at Cheddar ‘very solid and
large.’[191] Sometimes the same kind of variation is exhibited
by different species in the same locality. Thus specimens of _H.
aspersa_, _H. nemoralis_, and _H. hortensis_, taken from the same
bank at Torquay, presented a straw-coloured tinge of ground colour,
with red-brown bands or markings. Trochiform _H. nemoralis_ and _H.
arbustorum_, sinistral _H. hortensis_ and _H. aspersa_, sinistral _H.
aspersa_ and _H. virgata_, and similarly banded forms of _H. caperata_
and _H. virgata_, have been taken together.[192]
The immediate neighbourhood of the sea appears frequently to have the
effect of dwarfing land Mollusca. Thus the var. _conoidea_ of _Helix
aspersa_, which is small, conical, with a compressed mouth, occurs
‘on sandhills and cliffs at the seaside.’ The varieties _conica_ and
_nana_ of _Helix hispida_ are found ‘near the sea.’ _Helix virgata_
is exceedingly small in similar localities, and tends to become
unicoloured. _H. caperata_ var. _Gigaxii_, a small depressed form,
occurs at ‘Sandwich and Falmouth.’[193] Sometimes, however, the exact
opposite is the case, for _H. nemoralis_ var. _major_, which is
‘much larger’ than the type, occurs on ‘sandhills and downs’ and is
‘remarkably large in the I. of Arran, Co. Galway.’ The dwarf form of
_Limnaea peregra_ known as _maritima_ appears to be confined to the
neighbourhood of the sea.
Dwarfing of the shell seems frequently to be the result of an elevated
locality, not perhaps so much as the direct consequence of purer air
and less barometric pressure, as of changes in the character of the
food supply and in the humidity of the air. Several species of _Helix_
have a variety _minor_ which is characteristic of an Alpine habitat.
_Helix arbustorum_ var. _alpestris_, which is scarcely two-thirds the
size of the type, occurs on the Swiss Alps in the region of perpetual
snow. Sometimes a very slight elevation is sufficient to produce the
dwarfed form. At Tenby the type form of _Helix pisana_ is scattered in
countless numbers over the sandhills just above high-water mark. At the
extreme western end of these sandhills rises abruptly to a height of
over 100 feet the promontory known as Giltar Head, the vegetation of
which is entirely distinct from that of the burrows below. There is a
colony of _H. pisana_ at the end of Giltar, all of which are devoid of
the characteristic markings of the typical form, and most are dwarfed
and stunted in growth.
Occasionally the same variety will be found to be produced by
surroundings of very different nature. Thus the var. _alpestris_, of
_H. arbustorum_ mentioned above, besides being characteristic of high
Alpine localities, also occurs abundantly in low marshes at Hoddesdon
on the River Lea. _Helix pulchella_ var. _costata_, according to
Jeffreys, is found in dry and sandy places, often under loose stones
and bricks on walls, while other authorities have noticed it in wet and
dry localities quite indifferently.
Sometimes the production of a variety may be traced to the intrusion
of some other organism. According to Brot, nine-tenths of the _Limnaea
peregra_ inhabiting a certain pond near Geneva, were, during one
season, afflicted with a malformation of the base of the columella.
This deformity coincided with the appearance, in the same waters, of
extraordinary numbers of _Hydra viridis_. The next season, when the
_Hydra_ disappeared, the next generation of _Limnaea_ was found to
have resumed its normal form.
It has been noticed that a form of _Helix caperata_ with a flattened
spire and wide umbilicus is restricted to tilled fields, especially the
borders of clover fields, while a form with a more elevated spire and
more compact whorls occurs exclusively in open downs and uncultivated
places. The Rev. S. S. Pearce accounts[194] for this divergence by the
explanation that the flatter spire enables the shell of the fields
to creep about more easily under the leaves or matted weeds, seldom
requiring to crawl up a stalk or stem, while on the short turf of the
downs and pastures the smaller and more rounded shell enables the
animal to manoeuvre in and out of the blades of grass, and even to
crawl up them with considerable activity. The same writer endeavours
to explain the causes which regulate the distribution of _H. caperata_
var. _ornata_. He found that this variety (dark bands on a white
ground) occurred almost exclusively on downs which were fed upon by
sheep, associated with the common or mottled form, while the latter
form alone occurred in localities where sheep were not accustomed to
feed. Assuming then, as is probably the case, that sheep, in the course
of their close pasturing, devour many small snails, he believes that
individuals of the more conspicuous form _ornata_ were more likely
to be noticed, and therefore avoided, by the sheep, than the mottled
form, which would more easily escape their observation. Hence the var.
_ornata_ is due to the advantage which strikingly coloured individuals
obtained owing to their conspicuous habit, as compared with the typical
form, which would be less readily detected.
(_b_) _Changes in Soil, Station, Character of Water, etc._--A
deficiency of lime in the composition of the soil of any particular
locality produces very marked effects upon the shells of the Mollusca
which inhabit it; they become small and very thin, occasionally almost
transparent. The well-known var. _tenuis_ of _Helix aspersa_ occurs
on downs in the Channel Islands where calcareous material is scarce.
For similar reasons, _H. arbustorum_ develops a var. _fusca_, which is
depressed, very thin, and transparent, at Scilly, and also at Lunna I.,
E. Zetland.
[Illustration: FIG. 35.--19 specimens of _Purpura lapillus_ L.,
Great Britain, illustrating variation.
(1) Felixstowe, sheltered coast; (2), (3) Newquay, on veined
and coloured rock; (4) Herm, rather exposed; (5) Solent, very
sheltered; (6) Land’s End, exposed rocks, small food supply;
(7) Scilly, exposed rocks, fair food supply; (8) St. Leonards,
flat mussel beds at extreme low water; (9) Robin Hood’s Bay,
sheltered under boulders, good food supply; (10) Rhoscolyn, on
oyster bed, 4–7 fath. (Macandrew); (11) Guernsey, rather exposed
rocks; (12) Estuary of Conway, very sheltered, abundant food
supply; (13), (14) Robin Hood’s Bay, very exposed rocks, poor
food supply; (14) slightly monstrous; (15), (16), (17) Morthoe,
rather exposed rocks, but abundant food supply; (18) St. Bride’s
Bay; (19) L. Swilly, sheltered, but small food supply. All from
the author’s collection, except (10).]
The common dog-whelk (_Purpura lapillus_) of our own coasts is an
exceedingly variable species, and in many cases the variations may be
shown to bear a direct relation to the manner of life (Fig. 35). Forms
occurring in very exposed situations, _e.g._ Land’s End, outer rocks of
the Scilly Is., coasts of N. Devon and Yorkshire, are stunted, with a
short spire and relatively large mouth, the latter being developed in
order to increase the power of adherence to the rock and consequently
of resistance to wave force. On the other hand, shells occurring in
sheltered situations, estuaries, narrow straits, or even on open coasts
where there is plenty of shelter from the waves, are comparatively
of great size, with a well-developed, sometimes produced spire, and
a mouth small in proportion to the area of shell surface. In the
accompanying figure, the specimens from the Conway estuary and the
Solent (12, 5) well illustrate this latter form of shell, while that
from exposed rocks is illustrated by the specimens from Robin Hood’s
Bay (13, 14). Had these specimens occurred alone, or had they been
brought from some distant and unexplored region, they must inevitably
have been described as two distinct species.
[Illustration: FIG. 36.--Valves of _Cardium edule_ from the four
upper terraces of Shumish Kul, a dry salt lake adjacent to the
Aral Sea. (After Bateson.)]
Mr. W. Bateson has made[195] some observations on the shells of
_Cardium edule_ taken from a series of terraces on the border of
certain salt lakes which once formed a portion of the Sea of Aral.
As these lakes gradually became dry, the water they contained became
salter, and thus the successive layers of dead shells deposited on
their borders form an interesting record of the progressive variation
of this species under conditions which, in one respect at least, can
be clearly appreciated. At the same time the diminishing volume of
water, and the increasing average temperature, would not be without
their effect. It was found that the principal changes were as follows:
the thickness, and consequently the weight, of the shells became
diminished, the size of the beaks was reduced, the shell became highly
coloured, and diminished considerably in size, and the breadth of the
shells increased in proportion to their length (Fig. 36). Shells of
the same species of _Cardium_, occurring in Lake Mareotis, were found
to exhibit very similar variations as regards colour, size, shape, and
thickness.
_Unio pictorum_ var. _compressa_ occurs near Norwich at two similar
localities six or seven miles distant from one another, under
circumstances which tend to show that similar conditions have produced
similar results. The form occurs where the river, by bending sharply
in horse-shoe shape, causes the current to rush across to the opposite
side and form an eddy near the bank on the outside of the bend. Just
at the edge of the sharp current next the eddy the shells are found,
the peculiar form being probably due to the current continually washing
away the soft particles of mud and compelling the shell to elongate
itself in order to keep partly buried at the bottom.[196]
The rivers Ouse and Foss, which unite just below York, are rivers
of strikingly different character, the Ouse being deep, rapid, with
a bare, stony bottom, and little vegetable growth, and receiving a
good deal of drainage, while the Foss is shallow, slow, muddy, full
of weeds and with very little drainage. In the Foss, fine specimens
of _Anodonta anatina_ occur, lustrous, with beautifully rayed shells.
A few yards off, in the Ouse, the same species of _Anodonta_ is dull
brown in colour, its interior clouded, the beaks and epidermis often
deeply eroded. Precisely the same contrast is shown in specimens of
_Unio tumidus_, taken from the same rivers, Ouse specimens being also
slightly curved in form. Just above Yearsley Lock in the Foss, _Unio
tumidus_ occurs, but always dwarfed and malformed, a result probably
due to the effect of rapidly running water upon a species accustomed to
live in still water.[197] Simroth records the occurrence of remarkably
distorted varieties in two species of _Aetheria_ which lived in swift
falls of the River Congo.[198]
A variety of _Limnaea peregra_ with a short spire and rather strong,
stoutly built shell occurs in Lakes Windermere, Derwentwater, and
Llyn-y-van-fach. It lives adhering to stones in places where there are
very few weeds, its shape enabling it to withstand the surf of these
large lakes, to which the ordinary form would probably succumb.[199]
Scalariform specimens of _Planorbis_ are said to occur most commonly in
waters which are choked by vegetation, and it has been shown that this
form of shell is able to make its way through masses of dense weed much
more readily than specimens of normal shape.
Continental authorities have long considered _Limnaea peregra_ and
_L. ovata_ as two distinct species. Hazay, however, has succeeded in
rearing specimens of so-called _peregra_ from the ova of _ovata_, and
so-called _ovata_ from the ova of _peregra_, simply by placing one
species in running water, and the other in still water.
According to Mr. J. S. Gibbons[200] certain species of _Littorina_, in
tropical and sub-tropical regions, are confined to water more or less
brackish, being incapable of living in pure salt water. “I have met,”
says Mr. Gibbons, “with three of these species, and in each case they
have been distinguished from the truly marine species by the extreme
(comparative) thinness of their shells, and by their colouring being
richer and more varied; they are also usually more elaborately marked.
They are to be met with under three different conditions--(1) in
harbours and bays where the water is salt with but a slight admixture
of fresh water; (2) in mangrove swamps where salt and fresh water mix
in pretty equal volume; (3) on dry land, but near a marsh or the dry
bed of one.
“_L. intermedia_ Reeve, a widely diffused E. African shell, attaches
itself by a thin pellicle of dried mucus to grass growing by the margin
of slightly brackish marshes near the coast, resembling in its mode
of suspension the Old World _Cyclostoma_. I have found it in vast
numbers in situations where, during the greater part of the year,
it is exposed to the full glare of an almost vertical sun, its only
source of moisture being a slight dew at night-time. The W. Indian _L.
angulifera_ Lam., and a beautifully coloured E. African species (_? L.
carinifera_), are found in mangrove swamps; they are, however, less
independent of salt water than the last.”
Mr. Gibbons goes on to note that brackish water species (although not
so solid as truly marine species) tend to become more solid as the
water they inhabit becomes less salt. This is a curious fact, and the
reverse of what one would expect. Specimens of _L. intermedia_ on
stakes at the mouth of the Lorenço Marques River, Delagoa Bay, are
much smaller, darker, and more fragile, than those living on grass a
few hundred yards away. _L. angulifera_ is unusually solid and heavy
at Puerto Plata (S. Domingo) among mangroves, where the water is in a
great measure fresh; at Havana and at Colon, where it lives on stakes
in water but slightly brackish, it is thinner and smaller and also
darker coloured.
(_c_) _Changes in the Volume of Water._--It has long been known that
the largest specimens, _e.g._ of _Limnaea stagnalis_ and _Anodonta
anatina_, only occurred in pieces of water of considerable size.
Recent observation, however, has shown conclusively that the volume of
water in which certain species live has a very close relation to the
actual size of their shells, besides producing other effects. _Lymnaea
megasoma_, when kept in an aquarium of limited size, deposited eggs
which hatched out; this process was continued in the same aquarium for
four generations in all, the form of the shell of the last generation
having become such that an experienced conchologist gave it as his
opinion that the first and last terms of the series could have no
possible specific relation to one another. The size of the shell
became greatly diminished, and in particular the spire became very
slender.[201]
The same species being again kept in an aquarium under similar
conditions, it was found that the third generation had a shell only
four-sevenths the length of their great grandparents. It was noticed
also that the sexual capacities of the animals changed as well. The
liver was greatly reduced, and the male organs were entirely lost.[202]
K. Semper conducted some well-known experiments bearing on this point.
He separated[203] specimens of _Limnaea stagnalis_ from the same mass
of eggs as soon as they were hatched, and placed them simultaneously in
bodies of water varying in volume from 100 to 2000 cubic centimetres.
All the other conditions of life, and especially the food supply, were
kept at the known optimum. He found, in the result, that the size of
the shell varied directly in proportion to the volume of the water
in which it lived, and that this was the case, whether an individual
specimen was kept alone in a given quantity of water, or shared it with
several others. At the close of 65 days the specimens raised in 100
cubic cm. of water were only 6 mm. long, those in 250 cubic cm. were 9
mm. long, those in 600 cubic cm. were 12 mm. long, while those kept in
2000 cubic cm. attained a length of 18 mm. (Fig. 37).
An interesting effect of a sudden fall of temperature was noticed by
Semper in connection with the above experiments. Vessels of unequal
size, containing specimens of the _Limnaea_, happened to stand before a
window at a time when the temperature suddenly fell to about 55° F. The
sun, which shone through the window, warmed the water in the smaller
vessels, but had no effect upon the temperature of the larger. The
result was, that the _Limnaea_ in 2000 cubic cm., which ought to have
been 10 mm. long when 25 days old, were scarcely longer, at the end of
that period, than those which had lived in the smaller vessels, but
whose water had been sufficiently warm.
[Illustration: FIG. 37.--Four equally old shells of _Limnaea
stagnalis_, hatched from the same mass of ova, but reared in
different volumes of water: =A= in 100, =B= in 250, =C= in 600,
and =D= in 2000 cubic centimetres. (After K. Semper.)]
CHAPTER IV
USES OF SHELLS FOR MONEY, ORNAMENT, AND FOOD--CULTIVATION OF THE
OYSTER, MUSSEL, AND SNAIL--SNAILS AS MEDICINE--PRICES GIVEN FOR
SHELLS
The employment of shells as a medium of exchange was exceedingly common
amongst uncivilised tribes in all parts of the world, and has by no
means yet become obsolete. One of the commonest species thus employed
is the ‘money cowry’ (_Cypraea moneta_, L.), which stands almost alone
in being used entire, while nearly all the other forms of shell money
are made out of portions of shells, thus requiring a certain amount of
labour in the process of formation.
One of the earliest mentions of the cowry as money occurs in an ancient
Hindoo treatise on mathematics, written in the seventh century A.D.
A question is propounded thus: ‘the ¼ of 1/16 of ⅕ of ¾ of ⅔ of ½ a
dramma was given to a beggar by one from whom he asked an alms; tell
me how many cowry shells the miser gave.’ In British India about 4000
are said to have passed for a shilling, but the value appears to differ
according to their condition, poor specimens being comparatively
worthless. According to Reeve[204] a gentleman residing at Cuttack is
said to have paid for the erection of his bungalow entirely in cowries.
The building cost him 4000 Rs. sicca (about £400), and as 64 cowries =
1 pice, and 64 pice = 1 rupee sicca, he paid over 16,000,000 cowries in
all.
Cowries are imported to England from India and other places for the
purposes of exportation to West Africa, to be exchanged for native
products. The trade, however, appears to be greatly on the decrease. At
the port of Lagos, in 1870, 50,000 cwts. of cowries were imported.[205]
A banded form of _Nerita polita_ was used as money in certain parts of
the South Pacific. The sandal-wood imported into the China market is
largely obtained from the New Hebrides, being purchased of the natives
in exchange for _Ovulum angulosum_, which they especially esteem as an
ornament. Sometimes, as in the Duke of York group, the use of shell
money is specially restricted to certain kinds of purchase, being
employed there only in the buying of swine.
Among the tribes of the North-West coasts of America the common
_Dentalium indianorum_ used to form the standard of value, until it
was superseded, under the auspices of the Hudson’s Bay Company, by
blankets. A slave was valued at a fathom of from 25 to 40 of these
shells, strung lengthwise. Inferior or broken specimens were strung
together in a similar way, but were less highly esteemed; they
corresponded more to our silver and copper coins, while the strings of
the best shells represented gold.
The _wampum_ of the eastern coast of North America differed from all
these forms of shell money, in that it required a laborious process
for its manufacture. Wampum consisted of strings of cylindrical beads,
each about a quarter of an inch in length and half that breadth. The
beads were of two colours, white and purple, the latter being the more
valuable. Both were formed from the common clam, _Venus mercenaria_,
the valves of which are often stained with purple at the lower margins,
while the rest of the shell is white. Cut small, ground down, and
pierced, these shells were converted into money, which appears to have
been current along the whole sea-board of North America from Maine to
Florida, and on the Gulf Coast as far as Central America, as well as
among the inland tribes east of the Mississippi. Another kind of wampum
was made from the shells of _Busycon carica_ and _B. perversum_. By
staining the wampum with various colours, and disposing these colours
in belts in various forms of arrangement, the Indians were able to
preserve records, send messages, and keep account of any kind of event,
treaty, or transaction.
Another common form of money in California was _Olivella biplicata_,
strung together by rubbing down the apex. Button-shaped disks cut from
_Saxidomus arata_ and _Pachydesma crassatelloides_, as well as oblong
pieces of _Haliotis_, were employed for the same purpose, when strung
together in lengths of several yards.
“There is a curious old custom,” writes Mr. W. Anderson Smith,[206]
“that used formerly to be in use in this locality [the western coast
of Scotland], and no doubt was generally employed along the sea-board,
as the most simple and ready means of arrangement of bargains by a
non-writing population. That was, when a bargain was made, each party
to the transaction got one half of a bivalve shell--such as mussel,
cockle, or oyster--and when the bargain was implemented, the half that
fitted exactly was delivered up as a receipt! Thus a man who had a box
full of unfitted shells might be either a creditor or a debtor; but the
box filled with fitted shells represented receipted accounts. Those who
know the difficulty of fitting the valves of some classes of bivalves
will readily acknowledge the value of this arrangement.”
Shells are employed for use and for ornament by savage--and even by
civilised--tribes in all parts of the world. The natives of Fiji
thread the large _Turbo argyrostoma_ and _crenulatus_ as weights at
the edge of their nets, and also employ them as sinkers. A _Cypraea
tigris_ cut into two halves and placed round a stone, with two or three
showy _Oliva_ at the sides, is used as a bait for cuttles. _Avicula
margaritifera_ is cut into scrapers and knives by this and several
other tribes. Breast ornaments of _Chama_, grouped with _Solarium
perspectivum_ and _Terebra duplicata_ are common among the Fijians, who
also mount the _Avicula_ on a backing of whales’ teeth sawn in two,
for the same purpose. The great Orange Cowry (_Cypraea aurantiaca_)
is used as a badge of high rank among the chieftains. One of the
most remarkable Fijian industries is the working of whales’ teeth to
represent this cowry, as well as the commoner _C. talpa_, which is more
easily imitated.
Among the Solomon islanders, cowries are used to ornament their shields
on great field days, and split cowries are worn as a necklace, to
represent human teeth. Small bunches of _Terebellum subulatum_ are
worn as earrings, and a large valve of _Avicula_ is employed as a head
ornament in the centre of a fillet. The same islanders ornament the
raised prows of their canoes, as well as the inside of the stern-post,
with a long row of single _Natica_.
The native Papuans employ shells for an immense variety of purposes.
Circlets for the head are formed of rows of _Nassa gibbosula_, rubbed
down till little but the mouth remains. Necklaces are worn which
consist of strings of _Oliva_, young _Avicula_, _Natica melanostoma_,
opercula of _Turbo_, and valves of a rich brown species of _Cardium_,
pendent at the end of strings of the seeds known as Job’s tears.
_Struthiolaria_ is rubbed down until nothing but the mouth is left,
and worn in strings round the neck. This is remarkable, since
_Struthiolaria_ is not a native Papuan shell, and indeed occurs no
nearer than New Zealand. Sections of _Melo_ are also worn as a breast
ornament, dependent from a necklace of cornelian stones. _Cypraea
erosa_ is used to ornament drinking bowls, and _Ovulum ovum_ is
attached to the native drums, at the base of a bunch of cassowary
feathers, as well as being fastened to the handle of a sago-beater.
In the same island, the great _Turbo_ and _Conus millepunctatus_ are
ground down to form bracelets, which are worn on the biceps. The
crimson lip of _Strombus luhuanus_ is cut into beads and perforated
for necklaces. Village elders are distinguished by a single _Ovulum
verrucosum_, worn in the centre of the forehead. The thick lip of
_Cassis cornuta_ is ground down to form nose pieces, 4½ inches long.
Fragments of a shell called _Kaïma_ (probably valves of a large
_Spondylus_) are worn suspended from the ears, with little wisps of
hair twisted up and thrust through a hole in the centre. For trumpets,
_Cassis cornuta_, _Triton tritonis_, and _Ranella lampas_ are used,
with a hole drilled as a mouthpiece in one of the upper whorls. Valves
of _Batissa_, _Unio_, and _Mytilus_ are used as knives for peeling
yams. Spoons for scooping the white from the cocoa-nut are made from
_Avicula margaritifera_. _Melo diadema_ is used as a baler in the
canoes.[207]
In the Sandwich Islands _Melampus luteus_ is worn as a necklace, as
well as in the Navigator Islands. A very striking necklace, in the
latter group, is formed of the apices of a _Nautilus_, rubbed down to
show the nacre. The New Zealanders use the green opercula of a Turbo,
a small species of _Venus_, and _Cypraea asellus_ to form the eyes of
their idols. Fish-hooks are made throughout the Pacific of the shells
of _Avicula_ and _Haliotis_, and are sometimes strengthened by a
backing made of the columella of _Cypraea arabica_. Small axe-heads are
made from _Terebra crenulata_ ground down (Woodlark I.), and larger
forms are fashioned from the giant _Tridacna_ (Fiji).
Shells are used to ornament the elaborate cloaks worn by the women of
rank in the Indian tribes of South America. Specimens of _Ampullaria_,
_Orthalicus_, _Labyrinthus_, and _Bulimulus_ depend from the bottom
and back of these garments, while great _Bulimi_, 6 inches long, are
worn as a breast ornament, and at the end of a string of beads and
teeth.[208]
The chank-shell (_Turbinella rapa_) is of especial interest from
its connexion with the religion of the Hindoos. The god Vishnu is
represented as holding this shell in his hand, and the sinistral form
of it, which is excessively rare, is regarded with extraordinary
veneration. The chank appears as a symbol on the coins of some of the
ancient Indian Empires, and is still retained on the coinage of the
Rajah of Travancore.
The chief fishery of the chank-shell is at Tuticorin, on the Gulf of
Manaar, and is conducted during the N. E. monsoon, October-May. In
1885–86 as many as 332,000 specimens were obtained, the net amount
realised being nearly Rs.24,000. In former days the trade was much
more lucrative, 4 or 5 millions of specimens being frequently shipped.
The government of Ceylon used to receive £4000 a year for licenses to
fish, but now the trade is free. The shells are brought up by divers
from 2 or 3 fathoms of water. In 1887 a sinistral specimen was found at
Jaffna, which sold for Rs.700.[209] Nearly all the shells are sent to
Dacca, where they are sliced into bangles and anklets to be worn by the
Hindoo women.
Perhaps the most important industry which deals only with the shells of
Mollusca is that connected with the ‘pearl-oyster.’ The history of the
trade forms a small literature in itself. It must be sufficient here
to note that the species in question is not an ‘oyster,’ properly so
called, but an _Avicula_ (_margaritifera_ Lam.). The ‘mother-of-pearl,’
which is extensively employed for the manufacture of buttons, studs,
knife-handles, fans, card-cases, brooches, boxes, and every kind of
inlaid work, is the internal nacreous laminae of the shell of this
species. The most important fisheries are those of the Am Islands, the
Soo-loo Archipelago, the Persian Gulf, the Red Sea, Queensland, and the
Pearl Islands in the Bay of Panama. The shell also occurs in several
of the groups of the South Pacific--the Paumotu, Gambier and Navigator
Islands, Tahiti being the centre of the trade--and also on the coasts
of Lower California.[210]
Pearls are the result of a disease in the animal of this species of
_Avicula_ and probably in all other species within which they occur.
When the _Avicula_ is large, well formed, and with ample space for
individual development, pearls scarcely occur at all, but when the
shells are crowded together, and become humped and distorted, as
well as affording cover for all kinds of marine worms and parasitic
creatures, then pearls are sure to be found. Pearls of inferior value
and size are also produced by _Placuna placenta_, many species of
_Pinna_, the great _Tridacna_, the common _Ostrea edulis_, and several
other marine bivalves. They are not uncommon in _Unio_ and _Anodonta_,
and the common _Margaritana margaritifera_ of our rapid streams is
still said to be collected, in some parts of Wales, for the purpose of
extracting its small ‘seed-pearls.’ Pink pearls are obtained from the
giant conch-shell of the West Indies (_Strombus gigas_), as well as
from certain _Turbinella_.
In Canton, many houses are illuminated almost entirely by skylights and
windows made of shells, probably the semitransparent valves of _Placuna
placenta_. In China lime is commonly made of ground cockle-shells,
and, when mixed with oil, forms an excellent putty, used for cementing
coffins, and in forming a surface for the frescoes with which the
gables of temples and private houses are adorned. Those who suffer from
cutaneous diseases, and convalescents from small-pox, are washed in
Canton with the water in which cockles have been boiled.[211]
A recent issue of the Peking Gazette contains a report from the
outgoing Viceroy of Fukhien, stating that he had handed over the
insignia of office to his successor, including _inter alia_ the
conch-shell bestowed by the Throne. A conch-shell with a whorl turning
to the _right_, _i.e._ a sinistral specimen, is supposed when blown to
have the effect of stilling the waves, and hence is bestowed by the
Emperor upon high officers whose duties oblige them to take voyages by
sea. The Viceroy of Fukhien probably possesses one of these shells in
virtue of his jurisdiction over Formosa, to which island periodical
visits are supposed to be made.[212]
Shells appear to be used occasionally by other species besides man.
Oyster-catchers at breeding time prepare a number of imitation nests
in the gravel on the spit of land where they build, putting bits of
white shell in them to represent eggs.[213] This looks like a trick
in order to conceal the position of the true nest. According to
Nordenskjöld, when the eider duck of Spitzbergen has only one or two
eggs in its nest, it places a shell of _Buccinum glaciale_ beside
them. The appropriation of old shells by hermit-crabs is a familiar
sight all over the world. Perhaps it is most striking in the tropics,
where it is really startling, at first experience, to meet--as I have
done--a large _Cassis_ or _Turbo_, walking about in a wood or on a hill
side at considerable distances from the sea. A Gephyrean (_Phascolion
strombi_) habitually establishes itself in the discarded shells of
marine Mollusca. Certain Hymenoptera make use of dead shells of _Helix
hortensis_ in which they build their cells.[214] Magnus believes that
in times when heavy rains prevail, and the usual insects do not venture
out, certain flowers are fertilised by snails and slugs crawling over
them, _e.g._ _Leucanthemum vulgare_ by _Limax laevis_.[215]
=Mollusca as Food for Man.=--Probably there are few countries in the
world in which less use is made of the Mollusca as a form of food than
in our own. There are scarcely ten native species which can be said to
be at all commonly employed for this purpose. Neighbouring countries
show us an example in this respect. The French, Italians, and Spanish
eat _Natica_, _Turbo_, _Triton_, and _Murex_, and, among bivalves,
_Donax_, _Venus_, _Lithodomus_, _Pholas_, _Tapes_, and _Cardita_,
as well as the smaller Cephalopoda. Under the general designation
of _clam_ the Americans eat _Venus mercenaria_, _Mya arenaria_,
and _Mactra solidissima_. In the Suez markets are exposed for sale
_Strombus_ and _Melongena_, _Avicula_ and _Cytherea_. At Panama _Donax_
and _Solen_ are delicacies, while the natives also eat the great
_Murex_ and _Pyrula_, and even the huge _Arca grandis_, which lives
embedded in the liquid river mud.
The common littoral bivalves seem to be eaten in nearly all countries
except our own, and it is therefore needless to enumerate them. The
Gasteropoda, whose habits are scarcely so cleanly, seem to require a
bolder spirit and less delicate palate to venture on their consumption.
The Malays of the East Indian islands eat _Telescopium fuscum_ and
_Pyrazus palustris_, which abound in the mangrove swamps. They throw
them on their wood fires, and when they are sufficiently cooked, break
off the top of the spire and suck the animal out through the opening.
_Haliotis_ they take out of the shell, string together, and dry in the
sun. The lower classes in the Philippines eat _Arca inaequivalvis_,
boiling them as we do mussels.[216] In the Corean islands a species
of _Monodonta_ and another of _Mytilus_ are quite peppery, and bite
the tongue; our own _Helix revelata_, as I can vouch from personal
experience, has a similar flavour. _Fusus colosseus_, _Rapana bezoar_,
and _Purpura luteostoma_ are eaten on the southern coasts of China;
_Strombus luhuanus_, _Turbo chrysostomus_, _Trochus niloticus_, and
_Patella testudinaria_, by the natives of New Caledonia; _Strombus
gigas_ and _Livona pica_ in the West Indies; _Turbo niger_ and
_Concholepas peruvianus_ on the Chilian coasts; four species of
_Strombus_ and _Nerita_, one each of _Purpura_ and _Turbo_, besides two
_Tridacna_ and one _Hippopus_, by the natives of British New Guinea.
West Indian negroes eat the large Chitons which are abundant on their
rocky coasts, cutting off and swallowing raw the fleshy foot, which
they call ‘beef,’ and rejecting the viscera. Dried cephalopods are a
favourite Chinese dish, and are regularly exported to San Francisco,
where the Chinamen make them into soup. The ‘Challenger’ obtained two
species of _Sepia_ and two of _Loligo_ from the market at Yokohama.
The insipidity of fresh-water Mollusca renders them much less desirable
as a form of food. Some species of Unionidae, however, are said to
be eaten in France. _Anodonta edulis_ is specially cultivated for
food in certain districts of China, and the African _Aetheriae_ are
eaten by negroes. _Navicella_ and _Neritina_ are eaten in Mauritius,
_Ampullaria_ and _Neritina_ in Guadeloupe, and _Paludina_ in Cambodia.
The vast heaps of empty shells known as ‘kitchen-middens,’ occur in
almost every part of the world. They are found in Scotland, Denmark,
the east and west coasts of North America, Brazil, Tierra del Fuego,
Australia and New Zealand, and are sometimes several hundred yards
in length. They are invariably composed of the edible shells of the
adjacent coast, mixed with bones of Mammals, birds, and fish. From
their great size, it is believed that many of them must have taken
centuries to form.
Pre-eminent among existing shell-fish industries stands the cultivation
of the oyster and the mussel, a more detailed account of which may
prove interesting.
The cultivation of the oyster[217] as a luxury of food dates at least
from the gastronomic age of Rome. Every one has heard of the epicure
whose taste was so educated that
“he could tell
At the first mouthful, if his oysters fed
On the Rutupian or the Lucrine bed
Or at Circeii.”[218]
The first artificial oyster-cultivator on a large scale appears to have
been a certain Roman named Sergius Orata, who lived about a century
B.C. His object, according to Pliny the elder,[219] was not
to please his own appetite so much as to make money by ministering
to the appetites of others. His _vivaria_ were situated on the
Lucrine Lake, near Baiae, and the Lucrine oysters obtained under his
cultivation a notoriety which they never entirely lost, although
British oysters eventually came to be more highly esteemed. He must
have been a great enthusiast in his trade, for on one occasion when he
became involved in a law-suit with one of the riparian proprietors,
his counsel declared that Orata’s opponent made a great mistake if he
expected to damp his ardour by expelling him from the lake, for, sooner
than not grow oysters at all, he would grow them upon the roof of his
house.[220] Orata’s successors in the business seem to have understood
the secret of planting young oysters in new beds, for we are told that
specimens brought from Brundisium and even from Britain were placed for
a while in the Lucrine Lake, to fatten after their long journey, and
also to acquire the esteemed “Lucrine flavour.”
Oysters are ‘in season’ whenever there is an ‘r’ in the month, in other
words, from September to April. ‘Mensibus erratis,’ as the poet has
it, ‘vos ostrea manducatis!’ It has been computed that the quantity
annually produced in Great Britain amounts to no less than sixteen
hundred million, while in America the number is estimated at five
thousand five hundred million, the value being over thirteen million
dollars, and the number of persons employed fifty thousand. Arcachon,
one of the principal French oyster-parks, has nearly 10,000 acres of
oyster beds, the annual value being from eight to ten million francs;
in 1884–85, 178,359,000 oysters were exported from this place alone. In
the season 1889–90, 50,000 tons of oysters were consumed in London.
Few will now be found to echo the poet Gay’s opinion:
“That man had sure a palate covered o’er
With brass or steel, that on the rocky shore
First broke the oozy oyster’s pearly coat,
And risq’d the living morsel down his throat.”
There were halcyon days in England once, when oysters were to be
procured at 8d. the bushel. Now it costs exactly that amount before
a bushel, brought up the Thames, can even be exposed for sale at
Billingsgate (4d. porterage, 4d. market toll), and prime Whitstable
natives average from 3½d. to 4d. each. The principal causes of this
rise in prices, apart from the increased demand, are (1) over-dredging;
(2) ignorant cultivation, and to these may be added (3) the effect of
bad seasons in destroying young oysters, or preventing the spat from
maturing. Our own principal beds are those at Whitstable, Rochester,
Colchester, Milton (famous for its ‘melting’ natives), Faversham,
Queenborough, Burnham, Poole, and Carlingford in Co. Down, and
Newhaven, near Edinburgh.
The oyster-farms at Whitstable, public and private, extend over an
area of more than 27 square miles. The principal of these is a kind of
joint-stock company, with no other privilege of entrance except birth
as a free dredgeman of the town. When a holder dies, his interest dies
with him. Twelve directors, known as “the Jury,” manage the affairs of
the company, which finds employment for several thousand people, and
sometimes turns over as much as £200,000 a year. The term ‘Natives,’
as applied to these Whitstable or to other English oysters, requires a
word of explanation. A ‘Native’ oyster is simply an oyster which has
been bred on or near the Thames estuary, but very probably it may be
developed from a brood which came from Scotland or some other place at
a distance. For some unexplained reason, oysters bred on the London
clay acquire a greater delicacy of flavour than elsewhere. The company
pay large sums for brood to stock their own grounds, since there can be
no certainty that the spat from their own oysters will fall favourably,
or even within their own domains at all. Besides purchases from other
beds, the parks are largely stocked with small oysters picked up along
the coast or dredged from grounds public to all, sometimes as much
as 50s. a bushel being paid for the best brood. It is probably this
system of transplanting, combined with systematic working of the beds,
which has made the Whitstable oyster so excellent both as to quality
and quantity of flesh. The whole surface of the ‘layings’ is explored
every year by the dredge, successive portions of the ground being
gone over in regular rotation, and every provision being made for the
well-being of the crop, and the destruction of their enemies. For three
days of every week the men dredge for ‘planting,’ _i.e._ for the
transference of suitable specimens from one place to another, the
separation of adhering shells, the removal of odd valves and of every
kind of refuse, and the killing off of dangerous foes. On the other
three days they dredge for the market, taking care only to lift such a
number as will match the demand.
The Colne beds are natural beds, as opposed to the majority of the
great working beds, which are artificial. They are the property of
the town of Colchester, which appoints a water-bailiff to manage the
concern. Under his direction is a jury of twelve, who regulate the
times of dredging, the price at which sales are to be made, and are
generally responsible for the practical working of the trade. Here, and
at Faversham, Queenborough, Rochester, and other places, ‘natives’ are
grown which rival those of Whitstable.
There can be no question, however, that the cultivation of oysters by
the French is far more complete and efficient than our own, and has
reached a higher degree of scientific perfection combined with economy
and solid profits. And yet, between 40 and 50 years ago, the French
beds were utterly exhausted and unproductive, and showed every sign
of failure and decay. It was in 1858 that the celebrated beds on the
Ile de Ré, near Rochelle, were first started. Their originator was
a certain shrewd stone-mason, by name Boeuf. He determined to try,
entirely on his own account, whether oysters could not be made to grow
on the long muddy fore-shore which is left by the ebb of the tide.
Accordingly, he constructed with his own hands a small basin enclosed
by a low wall, and placed at the bottom a number of stones picked out
of the surrounding mud, stocking his ‘parc’ with a few bushels of
healthy young brood. The experiment was entirely successful, in spite
of the jeers of his neighbours, and Boeuf’s profits, which soon began
to mount up at an astonishing rate, induced others to start similar
or more extensive farms for themselves. The movement spread rapidly,
and in a few years a stretch of miles of unproductive mud banks was
converted into the seat of a most prosperous industry. The general
interests of the trade appear to be regulated in a similar manner to
that at Whitstable; delegates are appointed by the various communities
to watch over the business as a whole, while questions affecting the
well-being of oyster-culture are discussed in a sort of representative
assembly.
At the same time as Boeuf was planting his first oysters on the shores
of the Ile de Ré, M. Coste had been reporting to the French government
in favour of such a system of ostreiculture as was then practised
by the Italians in the old classic Lakes Avernus and Lucrinus. The
principle there adopted was to prevent, as far as possible, the escape
of the spat from the ground at the time when it is first emitted by
the breeding oyster. Stakes and fascines of wood were placed in such
a position as to catch the spat and give it a chance of obtaining a
hold before it perished or was carried away into the open sea. The old
oyster beds in the Bay of St. Brieuc were renewed on this principle,
banks being constructed and overlaid with bundles of wood to prevent
the escape of the new spat. The attempt was entirely successful, and
led to the establishment or re-establishment of those numerous parcs,
with which the French coast is studded from Brest to the Gironde. The
principal centres of the industry are Arcachon, Auray, Cancale, and la
Teste.
It is at Marennes, in Normandy, that the production of the celebrated
‘green oyster’ is carried out, that especial luxury of the French
epicure. Green oysters are a peculiarly French taste, and, though they
sometimes occur on the Essex marshes, there is no market for them in
England. The preference for them, on the continent, may be traced back
as early as 1713, when we find a record of their having been served
up at a supper given by an ambassador at the Hague. Green oysters are
not always green, it is only after they are placed in the ‘claires,’
or fattening ponds, that they acquire the hue; they never occur in
the open sea. The green colour does not extend over the whole animal,
but is found only in the branchiae and labial tentacles, which are
of a deep blue-green. Various theories have been started to explain
the ‘greening’ of the mollusc; the presence of copper in the tanks,
the chlorophyll of marine algae, an overgrowth of some parasite, a
disease akin to liver complaint, have all found their advocates. Prof.
Lankester seems to have established[221] the fact,--which indeed had
been observed 70 years before by a M. Gaillon,--that the greening is
due to the growth of a certain diatom (_Navicula ostrearia_) in the
water of the tanks. This diatom, which is of a deep blue-green colour,
appears from April to June, and in September. The oyster swallows
quantities of the _Navicula_; the pigment enters the blood in a
condition of chemical modification, which makes it colourless in all
the other parts of the body, but when the blood reaches the gills the
action of the secretion cells causes the blue tint to be restored. The
fact that the colour is rather green than blue in the gills, which are
yellowish brown, is due to certain optical conditions.
Not till the young white oyster has been steeped for several years in
the muddy waters of the ‘claires’ does it acquire the proper tint to
qualify it for the Parisian restaurant. The ‘claires’ are each about
100 feet square, surrounded by low broad banks of earth, about 3 feet
high and 6 feet thick at the base. Before the oysters are laid down,
the gates which admit the tide are carefully opened and shut a great
many times, in order to collect a sufficient amount of the _Navicula_.
When this is done, the beds are formed, and are not again overflowed
by the sea, except at very high tides. The oysters are shifted from
one ‘claire’ to another, in order to perfect the ‘greening’ process.
About fifty million of these ‘huitres de Marennes’ are produced
annually, yielding a revenue of 2,500,000 francs.
It appears, from the experience of one of the most enthusiastic of
French oyster-growers (Dr. Kemmerer), that oysters _grow_ best in
muddy water, and _breed_ best in clear water. Thus the open sea is
the place where the spat should fall and be secured, and, as soon as
it is of a suitable size, it should be transferred to the closed tank
or reservoir, where it will find the quiet and the food (confervae,
infusoria, minute algae) which are so requisite for its proper growth.
In muddy ground the animal and phosphorous matter increases, and the
flesh becomes fatter and more oily. A sudden change from the clear
sea-water to the muddy tank is inadvisable, and thus a series of
shiftings through tanks with water of graduated degrees of nourishment
is the secret of proper oyster cultivation.
The American oyster trade is larger even than the French. The Baltimore
oyster beds in the Chesapeake River and its tributaries cover 3000
acres, and produce an annual crop of 25 million bushels, as many as
100,000 bushels being sometimes taken from Chesapeake Bay in a single
day. Baltimore is the centre of the tinned oyster trade, while that
in raw oysters centres in New York. Most of the beds whose produce is
carried to New York are situated in New Jersey, Connecticut, Delaware,
or Virginia. The laws of these states do not allow the beds to be owned
by any but resident owners, and the New York dealers have consequently
to form fictitious partnerships with residents near the various oyster
beds, supply them with money to buy the beds and plant the oysters,
and then give them a share in the profits. It has been estimated that
from the Virginia beds 4,000,000 bushels of oysters are carried every
year to Fair Haven in New England, 4,000,000 to New York, 3,000,000 to
Providence, and 2,000,000 each to Boston, Philadelphia, and Baltimore.
The American ‘native’ (_O. virginica_) is a distinct species from our
own, being much larger and longer in proportion to its breadth; it is
said to be also much more prolific.
According to Milne-Edwards,[222] in the great oyster parks on the
coasts of Calvados, the oysters are educated to keep their shells
closed when out of water, and so retain water enough inside to keep
their gills moist, and arrive at their destination in good condition.
As soon as an oyster is taken out of the sea, it closes its shells,
and keeps them closed until the shock of removal has passed away, or
perhaps until the desirability of a fresh supply of water suggests
itself. The men take advantage of this to exercise the oysters,
removing them from the sea for longer and longer periods. In time this
has the desired effect; the well-educated mollusc learns that it is
hopeless to ‘open’ when out of the water, and so keeps his shell closed
and his gills moist, and his general economy in good condition.
Oysters have been known to live entirely out of water for a
considerable time. Prof. Verrill once noticed[223] a large cluster
of oysters attached to an old boot, hanging outside a fish-shop
in Washington. They had been taken out of the water on about 10th
December, and on 25th February following some of the largest were still
alive. It was noticed that all those which survived had the hinge
upward and the ventral edge downward, this being the most favourable
position possible for the retention of water within the gill-cavity,
since the edge of the mantle would pack against the margins of the
shell, and prevent the water from leaking away.
Such a succulent creature as the oyster has naturally many enemies.
One of the worst of these is the ravenous Starfish, or Five-finger.
His omnivorous capacities are well described by a clever writer and
shrewd observer of nature: “Here is one doubled up like a sea-urchin,
brilliant of hue, and when spread out quite 16 inches in diameter;
where, and oh where, can you obtain a prey? The hoe we carry is thrust
out and the mass dragged shorewards, when the rascal disgorges two
large dog-whelks he has been in the process of devouring. We feel
a comfortable glow of satisfaction to think that this enemy of our
oyster-beds is also the enemy of our other enemy, this carnivorous
borer. Here, quite close alongside, is another, only inferior in size,
and we drag him ashore likewise, to find that the fellow has actually
had the courage and audacity to suck the contents out of a large
horse-mussel (_Modiola_), the strong muscle alone remaining undevoured.
We proceed along but a short way when we meet with still another in
the curled-up condition in which they gorge themselves, and as we drag
it shorewards the shell of a _Tapes pullastra_ drops from the relaxing
grasp of the ogre. Slowly the extended stomach returns to its place,
and the monster settles back to an uncomfortable after-dinner siesta
on an exposed boulder; for the starfish wraps its turned-out stomach
around the prey it has secured, in place of attempting to devour the
limey covering in which most of its game is protected. Once the mouth
of the shell is enclosed in the stomach of the starfish, the creature
soon sickens, the hinge-spring relaxes its hold, and the shell opening
permits the starfish to suck out the gelatinous contents, and cast free
the calcareous skeleton.”[224]
According to other observers the starfish seizes the oyster with two
of his fingers, while with the other three he files away the edge of
the flat or upper valve until the points of contact with the round
valve are reduced almost to nothing; then he can introduce an arm, and
the rest is easy work. Others suggest that the starfish suffocates the
oyster by applying two of its fingers so closely to the edge of the
valves that the oyster is unable to open them; after a while the vital
powers relax and the shell gapes. The Rev. J. G. Wood holds[225] that
the starfish pours a secretion from its mouth which “paralyses the
hinge muscle and causes the shell to open.” Sometimes in a single night
a whole bed of oysters will be totally destroyed by an invasion of
starfish. Another dreaded enemy is the ‘whelk,’ a term which includes
_Purpura lapillus_, _Murex erinaceus_, _Buccinum undatum_, and probably
also _Nassa reticulata_. All these species perforate the shell with the
end of their radula, and then suck out the contents through the neatly
drilled hole. Skate fish are the cause of terrible destruction in the
open beds, and a scarcely less dangerous visitant is the octopus. Crabs
crush the young shells with their claws, and are said to gather in
bands and scratch sand or mud over the larger specimens, which makes
them open their shells. Yet another, and perhaps unconscious, foe is
found in the common mussel, which takes up room meant for the young
oysters, grows over the larger individuals, and harbours all sorts
of refuse between and under its closely packed ranks. _Cliona_, a
parasitic sponge, bores in between the layers of the oyster’s shell,
pitting them with tiny holes (corresponding to its _oscula_), and
disturbing the inmate, who has constantly to construct new layers of
shell from the inside. Weed, annelids, ‘blubber,’ shifting sand or
mud, sewage or any poisoning of the water, are seriously harmful to
the oyster’s best interests. A very severe winter is often the cause
of wholesale destruction in the beds. According to the _Daily News_ of
26th March 1891, the Whitstable oyster companies lost property to the
value of £30,000 in the exceptionally cold winter of 1890–91, when, on
the coast of Kent, the surface temperature of the sea sank below 32°,
and the advancing tide pushed a small ice-floe before it. Two million
oysters were laid down in one week of the following spring, to make up
for the loss. During the severe winter of 1892–93 extraordinary efforts
were made at Hayling I. to protect the oysters from the frost. Twenty
million oysters were placed in ponds for the winter, and a steam-engine
was for days employed to keep the ponds thawed and supplied with water,
while large coal and coke fires were kept burning at the edge of the
ponds.[226] On the other hand, the unusually warm and sunny summer
of 1893 is said to have resulted in the finest fall of spat known in
Whitstable for fifty years.[227]
The _reproductive activity_ of the oyster is supposed to commence about
the third year. Careful research has shown[228] that the sexes in the
English oyster are not separate, but that each individual is male as
well as female, producing spermatozoa as well as ova in the same gland.
Here, however, two divergent views appear. Some authorities hold that
the oyster does not fecundate its own eggs, but that this operation is
performed by spermatozoa emitted by other specimens. It is believed
that, in each individual, the spermatozoa arrive at maturity first, and
that the ova are not produced until after the spermatozoa have been
emitted; thus the oyster is first male and then female, morphologically
hermaphrodite, but physiologically unisexual. Others are of opinion
that the oyster does fecundate its own eggs, ova being first produced,
and passed into the infrabranchial chamber--the ‘white-sick’ stage--and
then, after an interval, spermatozoa being formed and fecundating
these ova--the ‘black-sick’ stage. In this latter view the oyster
is first female and then male, and is, both morphologically and
physiologically, hermaphrodite. The old view, that ‘black-sick’ oysters
are the male, and ‘white-sick’ the female, is therefore quite incorrect.
The ova, in their earliest stage, consist of minute oval clusters of
globules floating in a transparent mucus. They pass from the ovary
into the gills and folds of the mantle, and are probably fecundated
within the excretory ducts of the ovary, before arriving in the mantle
chamber. In this stage the oyster is termed ‘white-sick.’ In about a
fortnight, as the course of development proceeds, the fertilised ova
become ciliated at one end (the so-called _veliger_ stage, p. 131),
and soon pigment appears in various parts of the embryos, giving them
a darker colour, which varies from grayish to blue, and thus the
white-sick oyster becomes ‘black-sick.’ When the black spat emerge,
they are still furnished with cilia for their free-swimming life. This
is of very short duration, for unless the embryo finds some suitable
ground on which to affix itself within forty-eight hours, it perishes.
As the spat escapes from the parent oyster, which slightly opens its
valves and blows the spat out in jets, it resembles a thick cloud in
the water, and is carried about at the mercy of wind and tide. April to
August are the usual spawning months, warm weather being apparently an
absolute necessity to secure the adhering of the spat. A temperature
of 65° to 72° F. seems requisite for their proper deposit. Thus on a
fine, warm day, with little wind or tide running, the spat will fall
near the parents and be safely secured, while in cold, blustering
weather it will certainly be carried off to a distance, and probably
be altogether lost. The number of young produced by each individual
has been variously estimated at from 300,000 to 60,000,000. Either
extreme seems enormous, but it must be remembered that besides climatal
dangers, hosts of enemies--other Mollusca, fish, and Crustacea--beset
the opening career of the young oyster.
As soon as the spat has safely ‘fallen,’ it adheres to some solid
object, and loses the cilia which were necessary for its swimming life.
It begins to grow rapidly, increasing from about 1/20 inch in diameter
to about the size of a threepenny piece in five or six months, and in
a year to one inch in diameter. Roughly speaking, the best guide to an
oyster’s age is its size; it is as many years old as it measures inches
across.
The oyster is at its prime at the age of five; its natural life is
supposed to be about ten years. The rings, or ‘shoots’ on a shell are
not--as is frequently supposed--marks of annual growth; cases have been
noticed where as many as three ‘shoots’ were made during the year.
An oyster is furnished, on the protruding edges of the mantle, with
pigmented spots which may be termed ‘visual organs,’ though they hardly
rise to the capacities and organisation of real ‘eyes.’ But there is no
doubt that they are sufficiently sensitive to the action of light to
enable the oyster to apprehend the approach of danger, and close his
doors accordingly. ‘How sensitive,’ notes Mr. W. Anderson Smith,[229]
‘the creatures are to the light above them; the shadow of the iron as
it passes overhead is instantaneously noted, and snap! the lips are
firmly closed.’
The geographical distribution of _Ostrea edulis_ extends from Tränen,
in Norway, close to the Arctic circle, to Gibraltar and certain parts
of the Mediterranean, Holland, and N. Germany to Heligoland, and the
western shores of Sleswick and Jutland. It occurs in Iceland, but does
not enter the Baltic, where attempts to colonise it have always failed.
Some authorities regard the Mediterranean form as a distinct species.
The literature of oyster-cookery may be passed over in silence. The
curious may care to refer to M. S. Lovell’s _Edible British Mollusks_,
where no less than thirty-nine different ways of dressing oysters
are enumerated. It may, however, be worth while to add a word on
the subject of _poisonous_ oysters. Cases have been known where a
particular batch of oysters has, for some reason, been fatal to those
who have partaken of them. It is possible that this may have been due,
in certain instances, to the presence of a superabundance of copper
in the oysters, and there is no doubt that the symptoms detailed have
often closely resembled those of copper poisoning. Cases of poisoning
have occurred at Rochefort through’ the importation of ‘green oysters’
from Falmouth. It would no doubt be dangerous ever to eat oysters
which had grown on the copper bottom of a ship. But copper is present,
in more or less minute quantities, in very many Mollusca, and it
is more probable that a certain form of slow decomposition in some
shell-fish develops an alkaloid poison which is more harmful to some
people than to others, just as some people can never digest any kind
of shell-fish.[230] These alkaloid developments from putrescence are
called ptomaines. In confirmation of this view, reference may be
made to a case, taken from an Indian Scientific Journal, in which an
officer, his wife, and household ate safely of a basket of oysters for
three days at almost every meal. The basket then passed out of their
hands, not yet exhausted of its contents, and a man who had already
eaten of these oysters at the officer’s table was afterwards poisoned
by some from the same basketful.
The cultivation of the common mussel (_Mytilus edulis_ L.) is not
practised in this country, although it is used as food in the natural
state of growth all round our coasts. The French appear to be the only
nation who go in for extensive mussel farming. The principal of these
establishments is at a little town called Esnaudes, not far from La
Rochelle, and within sight of the Ile de Ré and its celebrated oyster
parks. The secret of the cultivation consists in the employment of
‘bouchots,’ or tall hurdles, which are planted in the mud of the
fore-shore, and upon which the mussel (_la moule_, as the French call
it) grows. The method is said[231] to have been invented as long ago as
1235 by a shipwrecked Irishman named Walton. He used to hang a purse
net to stakes, in the hope of capturing sea birds. He found, however,
that the mussels which attached themselves to his stakes were a much
more easily attainable source of food, and he accordingly multiplied
his stakes, out of which the present ‘_bouchot_’ system has developed.
The shore is simply a stretch of liquid mud, and the bouchots are
arranged in shape like a single or double V, with the opening looking
towards the sea. The fishermen, in visiting the bouchots, glide about
over the mud in _piroques_ or light, flat-bottomed boats, propelling
them by shoving the mud with their feet. Each bouchot is now about 450
yards long, standing 6 feet out of the mud, making a strong wall of
solid basket-work, and as there are altogether at least 500 bouchots,
the total mussel-bearing length of wall is nearly 130 miles.
The mussel-spat affixes itself naturally to the bouchots nearest the
sea, in January and February. Towards May the planting begins. The
young mussels are scraped off these outermost bouchots, and placed
in small bags made of old canvas or netting, each bag holding a good
handful of the mussels. The bags are then fastened to some of the inner
bouchots, and the mussels soon attach themselves by their byssus, the
bag rotting and falling away. They hang in clusters, increasing rapidly
in size, and at the proper time are transplanted to bouchots farther
and farther up the tide level, the object being to bring the matured
animal as near as possible to the land when it is time for it to be
gathered. This process, which aims at keeping the mussel out of the
mud, while at the same time giving it all the nutrition that comes
from such a habitat, extends over about a year in the case of each
individual. Quality, rather than quantity, is the aim of the Esnaudes
boucholiers. The element of quantity, however, seems to come in when
we are told that each yard of the bouchots is calculated to yield a
cartload of mussels, value 6 francs, and that the whole annual revenue
is at least £52,000.
In this country, and especially in Scotland, mussels are largely used
as bait for long-line fishing. Of late years other substances have
rather tended to take the place of mussels, but within the last twenty
years, at Newhaven on the Firth of Forth, three and a half million
mussels were required annually to supply bait for four deep-sea craft
and sixteen smaller vessels. According to Ad. Meyer,[232] boughs of
trees are laid down in Kiel Bay, and taken up again, after three, four,
or five years, between December and March, when they are found covered
with fine mussels. The boughs are then sold, just as they are, by
weight, and the shell-fish sent into the interior of Germany.
Mussels are very sensitive to cold weather. In 1874, during an
easterly gale, 195 acres of mussels at Boston, in Lincolnshire, were
killed in a single night. They soon affix themselves to the bottom
of vessels that have lain for any length of time in harbour or near
the coast. The bottom of the _Great Eastern_ steamship was at one
time so thickly coated with mussels that it was estimated that a
vessel of 200 tons could have been laden from her. In some of our
low-lying coast districts mussels are a valuable protection against
inundation. “An action for trespass was brought some time ago for the
purpose of establishing the right of the lord of the manor to prevent
the inhabitants of Heacham from taking mussels from the sea-shore.
The locality is the fore-shore of the sea, running from Lynn in a
north-westerly direction towards Hunstanton in Norfolk; and the
nature of the shore is such that it requires constant attention, and
no little expenditure of money, to maintain its integrity, and guard
against the serious danger of inundations of the sea. Beds of mussels
extend for miles along the shore, attaching themselves to artificial
jetties running into the sea, thereby rendering them firm, and thus
acting as barriers against the sea [and as traps to catch the silt,
and thus constantly raise the level of the shore]. Therefore, while it
is important for the inhabitants, who claim a right by custom, to take
mussels and other shell-fish from the shore, it is equally important
for the lord of the manor to do his utmost to prevent these natural
friends of his embankments and jetties from being removed in large
quantities.”[233]
The fable that Bideford Bridge is held together by the byssi of
_Mytilus_, which prevent the fabric from being carried away by the
tide, has so often been repeated that it is perhaps worth while to give
the exact state of the case, as ascertained from a Town Councillor.
The mussels are supposed to be of some advantage to the bridge,
consequently there is a by-law forbidding their removal, but the
corporation have not, and never had, any boat or men employed in any
way with regard to them.
Poisoning by mussels is much more frequent than by oysters. At
Wilhelmshaven,[234] in Germany, in 1885, large numbers of persons were
poisoned, and some died, from eating mussels taken from the harbour.
It was found that when transferred to open water these mussels became
innocuous, while, on the other hand, mussels from outside, placed in
the harbour, became poisonous. The cause obviously lay in the stagnant
and corrupted waters of the harbour, which were rarely freshened by
tides. It was proved to demonstration that the poison was not due to
decomposition; the liver of the mussels was the poisonous part. In the
persons affected, the symptoms were of three kinds, exanthematous
(skin eruptions), choleraic, and paralytic. Cases of similar poisoning
are not unfrequent in our own country, and the circumstances tend to
show that, besides the danger from mussels bred in stagnant water,
there is also risk in eating them when ‘out of season’ in the spawning
time.
_Whelks_ are very largely employed for bait, especially in the cod
fishery. The whelk fishery in Whitstable Bay, both for bait and for
human food, yields £12,000 a year. Dr. Johnston, of Berwick, estimated
that about 12 million limpets were annually consumed for bait in that
district alone. The cockle fishery in Carmarthen Bay employs from 500
to 600 families, and is worth £15,000 a year, that in Morecambe Bay is
worth £20,000.
=Cultivation of Snails for Food; Use as Medicine.=--It was a certain
Fulvius Hirpinus who, according to Pliny the elder,[235] first
instituted snail preserves at Tarquinium, about 50 B.C. He appears to
have bred several species in his ‘cochlearia,’ keeping them separate
from one another. In one division were the _albulae_, which came from
Reate; in another the ‘very big snails’ (probably _H. lucorum_), from
Illyria; in a third the African snails, whose characteristic was their
fecundity; in a fourth those from Soletum, noted for their ‘nobility.’
To increase the size of his snails, Hirpinus fed them on a fattening
mixture of meal and new wine, and, says the author in a burst of
enthusiasm, ‘the glory of this art was carried to such an extent that
a single snail-shell was capable of holding eighty sixpenny pieces.’
Varro[236] recommends that the snaileries be surrounded by a ditch, to
save the expense of a special slave to catch the runaways. Snails were
not regarded by the Romans as a particular luxury. Pliny the younger
reproaches[237] his friend Septicius Clarus for breaking a dinner
engagement with him, at which the _menu_ was to have been a lettuce,
three snails and two eggs apiece, barley water, mead and snow, olives,
beetroot, gourds and truffles, and going off somewhere else where he
got oysters, scallops, and sea-urchins. In Horace’s time they were used
as a gentle stimulant to the appetite, for
“’Tis best with roasted shrimps and Afric snails
To rouse your drinker when his vigour fails.”[238]
Escargotières, or snail-gardens, still exist in many parts of Europe,
_e.g._ at Dijon, at Troyes and many other places in central and
southern France, at Brunswick, Copenhagen, and Ulm. The markets at
Paris, Marseilles, Bordeaux, Toulouse, Nantes, etc., are chiefly
supplied by snails gathered from the open country, and particularly
from the vineyards, in some of which _Helix pomatia_ abounds. In the
_Morning Post_ of 8th May 1868 there is an account of the operation of
clearing the celebrated Clos de Vougeot vineyard of these creatures. No
less than 240 gallons were captured, at a cost in labour of over 100
francs, it being estimated that these snails would have damaged the
vines to an extent represented by the value of 15 to 20 pipes of wine,
against which may be set the price fetched by the snails when sold in
the market.
It is generally considered dangerous to eat snails at once which have
been gathered in the open country. Cases have occurred in which death
by poisoning has resulted from a neglect of this precaution, since
snails feed on all manner of noxious herbs. Before being sent to table
at the restaurants in the great towns, they are fattened by being fed
with bran in the same way as oysters.
The Roman Catholic Church permits the consumption of snails during
Lent. Very large numbers are eaten in France and Austria at this time.
At the village of Cauderon, near Bordeaux, it is the proper thing to
end Carnival with especial gaiety, but to temper the gaiety with a dish
of snails, as a foretaste of Lenten mortification.
The following species appear to be eaten in France at the present
day: _H. pomatia_, _aspersa_, _nemoralis_, _hortensis_, _aperta_,
_pisana_, _vermiculata_, _lactea_. According to Dr. Gray, the glassmen
at Newcastle used to indulge in a snail feast once a year, and a recent
writer informs us that _H. aspersa_ is still eaten by working people in
the vicinity of Pontefract and Knottingley.[239] But in this country
snails appear to be seldom consciously used as an article of food;
the limitation is necessary, for Lovell tells us that they are much
employed in the manufacture of cream, and that a retired (!) milkman
pronounced it the most successful imitation known.
Preparations made from snails used to be highly esteemed as a cure
for various kinds of diseases and injuries. Pliny the elder recommends
them for a cough and for a stomach-ache, but it is necessary “to take
an uneven number of them.”[240] Five African slugs, roasted and beaten
to a powder, with half a drachm of acacia, and taken with myrtle wine,
is an excellent remedy for dysentery. Treated in various ways, snails
have been considered, in modern times, a cure for ague, corns, web
in the eye, scorbutic affections, hectic fevers, pleurisy, asthma,
obstructions, dropsy, swelling of the joints, headache, an impostume
(whitlow), and burns. One of Pliny’s remedies for headache, which
competes with the bones of a vulture’s head or the brain of a crow or
an owl, is a plaister made of slugs with their heads cut off, which is
to be applied to the forehead. He regards slugs as immature snails,
whose growth is not yet complete (_nondum perfectae_). Lovell
states that “a large trade in snails is carried on for Covent Garden
market in the Lincolnshire fens, and that they are sold at 6d. per
quart, being much used for consumptive patients and weakly children.”
The custom still seems to linger on in some parts of the country. Mr.
E. Rundle, of the Royal Cornwall Infirmary, gives his experience in
the following terms: “I well remember, some twelve years since, an
individual living in an adjoining parish [near Truro] being pointed
out to me as ‘a snail or slug eater.’ He was a delicate looking man,
and said to be suffering from consumption. Last summer I saw this man,
and asked him whether the statement that he was a ‘snail eater’ was
true: he answered, ‘Yes, that he was ordered small white slugs--not
snails--and that up till recently he had consumed a dozen or more every
morning, and he believed they had done him good.’ There is also another
use to which the country people here put snails, and that is as an eye
application. I met with an instance a few weeks since, and much good
seemed to have followed the use.”[241]
A reverend Canon of the Church of England, whose name I am not
permitted to disclose, informs me that there was a belief among the
youth of his native town (Pontypool, in Monmouthshire) that young
slugs were ‘good for consumption,’ and that they were so recommended
by a doctor who practised in the town. The slugs selected were about ¾
inch long, “such as may be seen crawling on the turf of a hedge-bank
after a shower of rain.” They were “placed upon the tongue without
any previous preparation, and swallowed alive.” My informant himself
indulged in this practice for some time, “not on account of any
gustatory pleasure it afforded, but from some vague notion that it
might do him good.”
A colleague of mine at King’s College tells me that the country people
at Ponteland, near Morpeth, habitually collect _Limax agrestis_
and boil it in milk as a prophylactic against consumption. He has
himself frequently devoured them alive, but they must be swallowed, not
scrunched with the teeth, or they taste somewhat bitter.
Snails have occasionally fallen, with other noxious creatures, under
the ban of the Church. In a prayer of the holy martyr Trypho of
Lampsacus (about 10th cent. A.D.) there is a form of exorcism
given which may be used as occasion requires. It runs as follows: “O
ye Caterpillars, Worms, Beetles, Locusts, Grasshoppers, Woolly-Bears,
Wireworms, Longlegs, Ants, Lice, Bugs, Skippers, Cankerworms,
Palmerworms, _Snails_, Earwigs, and all other creatures that cling
to and wither the fruit of the grape and all other herbs, I charge you
by the many-eyed Cherubim, and by the six-winged Seraphim, which fly
round the throne, and by the holy Angels and all the Powers, etc. etc.,
hurt not the vines nor the land nor the fruit of the trees nor the
vegetables of ---- the servant of the Lord, but depart into the wild
mountains, into the unfruitful woods, in which God hath given you your
daily food.”
=Prices given for Shells.=--Very high prices have occasionally been
given for individual specimens, particularly about thirty or forty
years ago, when the mania for collecting was at its height. In those
days certain families, such as the Volutidae, Conidae, and Cypraeidae,
were the especial objects of a collector’s ardour, and he spared no
expense to make his set of the favourite genus as complete as possible.
Thus at Stevens’ auction-rooms in Covent Garden, on 21st July 1854,
one specimen of _Conus cedo nulli_ fetched £9: 10s., and another £16,
a _C. omaicus_ 16 guineas, _C. victor_ £10, and _C. gloria maris_,
the greatest prize of all, £43: 1s. At the Vernède sale, on 14th
Dec. 1859 two _Conus omaicus_ fetched £15 and £22, and a _C. gloria
maris_ £34. At the great Dennison sale, in April 1865, the _Conidae_
fetched extravagant prices, six specimens averaging over £20 apiece.
_Conus cedo nulli_ went for £18 and £22, _C. omaicus_ for £12, _C.
malaccanus_ for 10 guineas (this and one of the _cedo nulli_ being the
actual specimens figured in Reeve’s _Conchologia Iconica_), _C. cervus_
for £19 and _C. gloria maris_ for £42. On 9th May 1866 a _Cypraea
Broderipii_ was sold at Stevens’ auction-rooms for £13, and at the
Dennison sale a _Cypraea princeps_ fetched £40, and _C. guttata_ £42.
The _Volutidae_, although not quite touching these prices, have yet
done fairly well. Mr. Dennison’s _Voluta fusiformis_ sold for £6: 15s.,
_V. papillaris_ for £5, _V. cymbiola_ for £5: 15s., _V. reticulata_
for eight guineas, and two specimens of the rarest of all Volutas, _V.
festiva_, for £14 and £16, both being figured in the _Conchologia_. At
the same sale, two unique specimens of _Oniscia Dennisoni_ fetched £17
and £18 respectively, and, at the Vernède sale, _Ancillaria Vernèdei_
was bought for £6: 10s., and _Voluta piperata_ for £7: 10s.
A unique specimen of a recent _Pleurotomaria_ (_quoyana_ F. and B.) was
purchased by Miss de Burgh in 1873 for 25 guineas, and another species
of the same genus (_adansoniana_ Cr. and F.), of extraordinary size and
beauty, is now offered for sale for about £100.
Bivalves have never fetched quite such high prices as univalves, but
some of the favourite and showy genera have gone near to rival them.
On 22nd June 1869, at Stevens’, _Pecten solaris_ fetched £4: 5s.,
_P. Reevii_ £4: 8s., and _Cardita varia_ 5 guineas. Mr. Dennison’s
specimens of _Pecten subnodosus_ sold for £7, of _Corbula Sowerbyi_ for
£10, of _Pholadomya candida_ for £8 and £13, while at the Vernède sale
a _Chama damicornis_ fetched £7.
CHAPTER V
REPRODUCTION--DEPOSITION OF EGGS--DEVELOPMENT OF THE FERTILISED
OVUM--DIFFERENCES OF SEX--DIOECIOUS AND HERMAPHRODITE
MOLLUSCA--DEVELOPMENT OF FRESH-WATER BIVALVES
Reproduction in the Mollusca invariably takes place by means of
eggs, which, after being developed in the ovary of the female, are
fertilised by the spermatozoa of the male. As a rule, the eggs are
‘laid,’ and undergo their subsequent development apart from the
parent. This rule, however, has its exceptions, both among univalve
and bivalve Mollusca, a certain number of which hatch their young
from the egg before expelling them. Such ovoviviparous genera are
_Melania_, _Paludina_, _Balea_, and _Coeliaxis_ among land and
fresh-water Mollusca, and _Cymba_ and many _Littorina_ amongst marine.
The young of _Melania tuberculata_, in Algeria, have been noticed to
return, as if for shelter, to the branchial cavity of the mother, some
days after first quitting it. Isolated species among Pulmonata are
known to be ovoviviparous, _e.g._ _Patula Cooperi_, _P. Hemphilli_,
and _P. rupestris_, _Acanthinula harpa_, _Microphysa vortex_,
_Pupa cylindracea_ and _muscorum_, _Clausilia ventricosa_, _Opeas
dominicensis_, _Rhytida inaequalis_, etc. All fresh-water Pelecypoda
yet examined, except _Dreissensia_, are ovoviviparous.
The number of eggs varies greatly, being highest in the Pelecypoda. In
_Ostrea edulis_ it has been estimated at from 300,000 to 60,000,000;
in _Anodonta_ from 14,000 to 20,000; in _Unio pictorum_ 200,000. The
eggs of _Doris_ are reckoned at from 80,000 to 600,000, of _Loligo_
and _Sepia_ at about 30,000 to 40,000. Pulmonata lay comparatively few
eggs. _Arion ater_ has been observed to lay 477 in forty-eight days (p.
42). Nests of _Helix aspersa_ have been noticed, in which the number
of eggs varied from about 40 to 100. They are laid in little cup-shaped
hollows at the roots of grass, with a little loose earth spread over
them. The eggs of _Testacella_ are rather large, and very elastic; if
dropped on a stone floor they will rebound sharply several inches. The
_Cochlostyla_ of the Philippines lay their eggs at the tops of the
great forest trees, folding a leaf together to serve as a protection.
The eggs of the great tropical _Bulimus_ and _Achatina_, together with
those of the _Macroön_ group of _Helix_ (_Helicophanta_, _Acavus_,
_Panda_) are exceedingly large, and the number laid must be decidedly
less than in the smaller Pulmonata. _Bulimus oblongus_, for instance,
from Barbados, lays an egg about the size of a sparrow’s (Fig. 38),
_Achatina sinistrorsa_ as large as a pigeon’s. The Cingalese _Helix
Waltoni_ when first hatched is about the size of a full-grown _H.
hortensis_. There is, in the British Museum, a specimen of the egg of a
_Bulimus_ from S. America (probably _maximus_ or _popelairanus_) which
measures exactly 1¾ inch in length.
The _Limnaeidae_ deposit their eggs in irregular gelatinous masses
on the under side of the leaves of water-plants, and on all kinds of
débris.
[Illustration: FIG. 38.--Newly-hatched young and egg of _Bulimus
oblongus_ Müll., Barbados. _Natural size._]
The _Rachiglossa_ or marine carnivorous families lay their eggs in
tough leathery or bladdery capsules, which are frequently joined
together in shapes which differ with the genus. Each capsule contains
a varying number of ova. The cluster of egg-capsules of _Buccinum
undatum_ is a familiar object on all our sandy coasts. The capsules
of _Purpura lapillus_ are like delicate pink grains of rice, set
on tiny stalks. They are not attached to one another, but are set
closely together in groups in sheltered nooks of the rocks. A single
_Purpura_ has been observed to produce 245 capsules! _Busycon_ lays
disc-shaped capsules which are all attached at a point in the edge to
a cartilaginous band nearly 3 feet in length, looking like a number of
coins tied to a string at equal distances from one another. In _Murex
erinaceus_ the egg-capsules are triangular, with a short stalk. They
are deposited separately in clusters of from 15 to 150, there being
about 20 ova in each capsule. It appears that all the species of the
same genus have by no means the same method of depositing their eggs,
nor do they always produce eggs of at all similar size or shape. Thus,
of two British species of _Nassa_, _N. reticulata_ lays egg-capsules in
shape like flattened pouches with a short stalk, and fastens them in
rows to the leaves of _Zostera_; _M. incrassata_, on the other hand,
deposits solitary capsules, which are shaped like rounded oil-flasks.
_Neptunea antiqua_ lays its eggs in bunched capsules, like _Bucc.
undatum_ (Fig. 40), but the capsules of _N. gracilis_ are solitary.
[Illustration: FIG. 39.--Various forms of spawn in
_Prosobranchiata_: =A= and =D=, _Pyrula_ or _Busycon_; =B=,
_Conus_; =C=, _Voluta musica_; =E=, _Ampullaria_ (from specimens
in the British Museum); all × ⅔.]
In _Natica_ the eggs are deposited in what looks like a thick piece of
sand-paper, curled in a spiral form (Fig. 41). The sand is agglutinated
by copious mucus into a sort of sheet, and the eggs are let into this,
sometimes (_N. heros_) in regular quincunx form. _Ianthina_ attaches
its eggs to the under side of its float (Fig. 42). The Trochidae
deposit their eggs on the under side of stones and sea-weeds, each
ovum being contained in a separate capsule, and all the capsules
glued together into an irregular mass of varying size. The female of
_Galerus chinensis_ hatches her eggs by keeping them between her foot
and the stone she adheres to. They are laid in from 6 to 10 capsules,
connected by a pedicle and arranged like the petals of a rose, with
10 to 12 eggs in each capsule. Those _Littorina_ which are not
ovoviviparous deposit their spawn on sea-weeds, rocks, and stones. The
eggs are enveloped in a glairy mass which is just firm enough to retain
its shape in the water; each egg has its own globule of jelly and is
separated from the others by a very thin transparent membrane.[242]
[Illustration: FIG. 40.--Egg-capsules of, =A=, _Nassa reticulata_
L. × ⅔; =B=, _Buccinum undatum_ L. × ⅔; =C=, _Neptunea antiqua_
L. × ⅓.]
[Illustration: FIG. 41.--Spawn of a species of _Natica_ (from a
specimen in the British Museum) × ½.]
[Illustration: FIG. 42.--_Ianthina fragilis_ Lam. =FL=, float;
=O=, ova; =Pr=, proboscis; =Br=, branchiae; =F=, foot. (Quoy and
Gaimard.)]
_Chiton marginatus_, when kept in captivity, has been noticed[243] to
elevate the posterior part of the girdle, and to pour out a continuous
stream of flaky white matter like a fleecy cloud, which proved to be of
a glutinous nature. It then discharged ova, at the rate of one or two
every second, for at least fifteen minutes, making a total of 1300 to
1500, each being about 1/100 inch diameter. The ova were shot into the
glutinous cloud, which seemed to serve as a sort of nidus to entangle
the ova and prevent them being carried away. The subsequent development
was rapid, and in seven days the young _Chiton_ was hatched, being then
about 1/20 inch long. Lovén has described the same species as laying
its eggs, loosely united in clusters of seven to sixteen, upon small
stones. There is probably some mistake about the identification, but
the observation illustrates the varying methods of oviposition among
allied forms.
[Illustration: FIG. 43.--Egg-capsules of =A=, _Sepia elegans_
Orb., and =B=, _Octopus vulgaris_ Lam.]
Not very much is known with regard to the ovipositing of the
_Cephalopoda_, especially those which inhabit deep water. Masses of
ova arranged in very various forms have occasionally been met with
floating in the ocean, but it is next to impossible to determine to
what species, or even genus, they belong.[244]
In _Loligo punctata_ the ova are contained in small cylindrical cases
measuring 3 to 4 in. by ½ in., to the number of about 250 ova in each
case. Hundreds of these cases are attached together like a bundle of
sausages or young carrots, and the movements of the embryos within can
be distinctly noted. _Sepia officinalis_ lays large black pear-shaped
capsules, each of which is tied to some place of attachment by a kind
of ribbon at the upper end of the capsule, the whole forming a large
group like a bunch of grapes. _Octopus vulgaris_ deposits thousands
of small berry-shaped ova, attached to a string which runs along the
centre of the mass (Fig. 43).
The so-called shell of the female _Argonauta_ is nothing more than a
form of protection for the ova, and is in no sense homologous to the
ordinary molluscan shell. The ova consist of a large granulated mass,
attached to a many branched stem; they are contained in the spire
of the shell, in contact with the posterior part of the body of the
mother, but sometimes project externally beyond the coil of the spire.
Certain species possess the curious property of laying their eggs
on the outside of their own shells. _Buccinopsis Dalei_ is not
unfrequently found decorated with its own egg-capsules. Possibly this
species, which lives on oozy ground, finds this the only secure place
of attachment for its progeny. _Neritina fluviatilis_ has a similar
habit, and so have many other species of _Neritina_ and _Navicella_.
It is not quite clear, in the latter cases, whether the eggs are laid
by the specimens on whose shell they are found, or whether they are
deposited by others. In either case, perhaps the shell is the safest
place for them in the rapid streams which both genera frequent.
Specimens of _Hydrobia ulvae_ taken on the wet sands at the mouth
of the Dee, are found to have several little rounded excrescences
scattered over the surface of the shell. These, on examination, are
found to be little masses of small sand-grains, in the centre of which
is a clear jelly containing segmenting ova or young embryos. Here
again, in all probability, the shell is the only comparatively stable
object, in the expanse of shifting sands, on which the eggs can be
laid.[245]
The pulmonate genus _Libera_, which occurs on a few of the island
groups in the Central Pacific, is remarkable for the habit of laying
its eggs within its own cavernous umbilicus, which is narrowed at the
lower part. The eggs number from four to six, or the same number of
very young shells may be seen closely packed in the cavity, each being
in shape exactly like a young _Planorbis_. This constriction of the
umbilicus does not occur till the formation of the last two whorls,
_i.e._ till the animal is sexually mature. Some species, but not all,
provide for the safety of their eggs more completely by forming a very
thin shelly plate, which nearly closes the umbilical region, and breaks
away or is absorbed to facilitate the escape of the young shells.[246]
=Union of Limax.=--With regard to the act of union itself, the method
in certain species of _Limax_ deserves special notice. _L. maximus_
has been observed at midnight to ascend a wall or some perpendicular
surface. A pair then crawl round and round one another emitting a
quantity of mucus which at length forms a patch, 2 to 2½ inches in
diameter. When this acquires consistency the pair begin to twist round
each other in corkscrew form, and detach themselves from the wall,
hanging by a cord of the thickened mucus, about 8–15 inches long, and
still twisting round each other. The external generative organs are
then protruded and copulation takes place, after which the bodies
untwist, separate, and crawl up the cord again to the wall.[247]
=Periodicity in Breeding.=--In the marine Mollusca, the winter months
appear to be the usual time for the deposition of eggs. Careful
observations have been made on the Mollusca occurring at Naples,[248]
and the general result seems to be that for all Orders alike the
six winter months from November to April, roughly speaking, are the
breeding time. Scarcely any forms appear to breed habitually in August,
September, or October. On our own coasts, Nudibranchiata come in shore
to deposit their ova from January to April. _Purpura lapillus_ may
be observed depositing ova all the year round, but is most active
from January to April. _Buccinum undatum_ breeds from October to May;
_Littorina_ all the year round.
The land Mollusca exhibit rather more periodicity than the marine. In
temperate climates they breed exclusively in the summer months. In the
tropics their periods are determined by the dry and rainy seasons,
where such occur, otherwise they cohabit all the year round. According
to Karl Semper, the snails of the warm Mediterranean region arrive at
sexual maturity when they are six months old, _i.e._ before they are
fully grown. After a rest of about three months during the heat of
summer, a second period of ovipositing occurs.[249] _Helix hortensis_
and _H. nemoralis_ ascend trees, sometimes to a height of forty feet,
when pairing.[250]
=Hybridism= as the result of union between different species of
Mollusca is exceedingly rare. Lecoq once noticed[251] on a wall at
Anduze (Gard) as many as twenty specimens of _Pupa cinerea_ united
with _Clausilia papillaris_. No offspring seem to have resulted from
what the professor calls ‘this innocent error,’ for the wall was
carefully scrutinised for a long time, and no hybrid forms were ever
detected.
The same observer noticed, in the Luxembourg garden at Paris, and
M. Gassies has noticed[252] at various occasions, union between
_Helix aspersa_ and _nemoralis_, _H. aspersa_ and _vermiculata_,
between _Stenogyra decollata_ and a _Helix_ (sp. not mentioned), _H.
variabilis_ and _pisana_, _H. nemoralis_ and _hortensis_. In the two
latter cases a hybrid progeny was the result. It has been noticed that
these unions generally took place when the air was in a very electric
condition, and rain had fallen, or was about to fall, abundantly.
Of marine species _Littorina rudis_ has been noticed[253] in union both
with _L. obtusata_ and with _L. littorea_, but no definite facts are
known as to the result of such unions.
=Self-impregnation= (see p. 44).
=Development of the Fertilised Ovum.=--The first stages in the
development of the Mollusca are identical with those which occur in
other classes of animals. The fertilised ovum consists of a vitellus
or yolk, which is surrounded with albumen, and is either contained in
a separate capsule, or else several, sometimes many, ova are found in
the same capsule, only a small proportion of which ultimately develop.
The germinal vesicle, which is situated at one side of the vitellus,
undergoes unequal segmentation, the result of which is usually the
formation of a layer of small ectoderm cells overlying a few much
larger cells which contain nearly the whole of the yolk. The large
cells are then invaginated, or are simply covered by the growth of the
ectoderm cells. The result in either case is the formation of an area,
the blastopore, where the inner cells are not covered by the ectoderm.
The blastopore gradually narrows to a circular opening, which, in
the great majority of cases, eventually becomes the mouth. The usual
differentiation of germinal layers takes place, the epiblast eventually
giving rise to the epidermis, nervous system, and special sense organs,
the hypoblast to the liver and to the middle region of the alimentary
tract, the mesoblast to the muscles, the body cavity, the vascular,
the excretory and reproductive systems. The next, or _trochosphere_
(_trochophora_) stage, involves the formation of a circlet of praeoral
cilia, dividing the still nearly spherical embryo into two unequal
portions, the smaller of which consists simply of the prostomium, or
part in front of the mouth, the larger bearing the mouth and anus.
So far the series of changes undergone by the embryo are not
peculiar to the Mollusca; we now come to those which are definitely
characteristic of that group. The stage next succeeding the development
of the trochosphere is the definitive formation of the _velum_, a
process especially characteristic of the Gasteropoda and Pelecypoda,
but apparently not occurring in the great majority of land Pulmonata.
[Illustration: FIG. 44.--Veligers of _Dentalium entalis_ L.: =A=,
longitudinal section of a larva 14 hours old, × 285; =B=, larva
of 37 hours, × 165; =C=, longitudinal section of larva of 34
hours, × 165; _m_, mouth; _v_, _v_, velum. (After Kowalewsky.)]
The circlet of cilia becomes pushed more and more towards the anterior
portion of the embryo, the cilia themselves become longer, while the
portion of the body from which they spring becomes elevated into a
ridge or ring, which, as a rule, develops on each side a more or less
pronounced lobe. The name _velum_ is applied to this entire
process of ciliated ring and lobes, and to the area which they enclose.
[Illustration: FIG.45.--Veliger of _Patella vulgata_ L., 130
hours old: _f_, rudimentary foot; _op_, operculum; _sh_, shell;
_v_, _v_, velum. (After Patten, highly magnified.)]
[Illustration: FIG. 46.--Developed larva of _Cyclas cornea_ L.:
_br_, rudimentary branchiae; _by_, byssus; _f_, foot; _m.e_,
mantle edge; _sh_, shell. (After Ziegler, highly magnified.)]
[Illustration: FIG. 47.--=A=, Advanced veliger of _Dreissensia_:
_f_, foot; _m_, mouth; _sh_, shell; _v_, _v_, velum. (After
Korschelt and Heider, much enlarged.) =B=, Veliger of a Pteropod
(_Tiedemannia_): _op_, operculum; _sh_, shell; _v_, velum. (After
Krohn, much enlarged.)]
In this so-called _veliger_ stage, the velum serves, in the first
place, to cause rotation of the larva within the egg-capsules, and,
after hatching, as an organ of locomotion. As a rule, the velum
disappears entirely in the adult mollusc after the free-swimming stage
is over, but in the common _Limnaea stagnalis_ it persists, losing its
cilia, as the very prominent circum-oral lobes. Simultaneously with
the development of the velum, and in some cases earlier, appear the
rudiments of the shell-gland and of the foot, the latter being situated
on the ventral side, between the mouth and anus, the former on the
dorsal side, behind the velum, and above the surface of the eventual
visceral sac. Thus the prime characteristics of the veliger stage,
subsequent to the appearance of the velum itself, are the development
of the visceral sac and shell-gland on the upper, and of the foot on
the under side. According to Lankester the primitive shell-gland does
not, as a rule, directly give rise to the shell of the adult mollusc,
but becomes filled up by a horny substance, and eventually disappears;
the permanent shell then forms over the surface of the visceral hump
from the original centre of the shell-gland. It is only in _Chiton_,
and possibly in _Limax_, that the primitive shell-sac is retained and
developed into the final shell-forming area, which is much wider, and
extends to the edges of the mantle. Within the velar area first appear
the rudiments of the tentacles and eyes; when these become developed
the velum atrophies and disappears.
Several of these veligers when captured in the open sea have been
mistaken for perfect forms, and have been described as such. Thus the
larva of _Dolium_ has been described as _Macgillivrayia_, that of a
_Purpura_ as _Chelotropis_ and _Sinusigera_, that of _Aporrhais pes
pelecani_ as _Chiropteron_, that of _Marsenia conspicua_ as _Brownia_,
_Echinospira_, and _Calcarella_.
_Cephalopoda._--The embryonic development of the Cephalopoda is
entirely distinct from that of all other Mollusca. The segmentation of
the vitellus is partial, and the embryo is furnished with a vitelline
sac, which is very large in the majority of cases (Fig. 48). There
is no free-swimming stage, but the embryo emerges from the egg fully
developed.
=Differences of Sex.=--In the Mollusca there are two main types of
sexual difference: (i) sexes separate (_dioecious_ type), (ii) sexes
united in the same individual (_hermaphrodite_ type).
[Illustration: FIG. 48.--Two stages in the development of _Loligo
vulgaris_ Lam.: _a_{1}_, _a_{1}_, first, and _a_{2}_, _a_{2}_,
second pairs of arms; _br_, branchiae, seen through _m_, mantle;
_e_, _e_, eyes; _fi_, fins; _fu_, funnel; _v.s_, vitelline sac.
(After Kowalewsky.)]
In some cases--_e.g._ certain Pelecypoda--what is practically a third
type occurs. The animal is hermaphrodite, but the male and female
elements are not developed simultaneously, _i.e._ the same individual
is at one time female, at another male.
1. The sexes are separate in
All _Cephalopoda_.
_Gasteropoda Amphineura_ (except _Neomeniidae_).
_Gasteropoda Prosobranchiata_ (except _Valvata_ and
some species of _Marsenia_).
_Scaphopoda_.
Many _Pelecypoda_.
2. The sexes are united in
_Gasteropoda Opisthobranchiata_.
_Gasteropoda Pulmonata_.
Certain _Pelecypoda_.[254]
In the dioecious Mollusca, sexual union is the rule, but is by no means
universal. In some instances,--_e.g._ _Vermetus_, _Magilus_, _Patella_,
_Haliotis_, _Crepidula_, _Chiton_, the _Scaphopoda_--the form and
habits of the animal do not admit of it; in others (many _Trochus_)
a male copulative organ is wanting. When this is the case, the male
scatters the spermatozoa freely; the majority must perish, but some
will be carried by currents in the direction of the female.
When the sexes are separate, the female is frequently larger than the
male. This is markedly the case in _Littorina_, _Buccinum_, and all the
_Cephalopoda_; in _Argonauta_ the difference is extreme, the male not
being more than ¼ the size of the female.
Those hermaphrodite Mollusca which are capable of sexual union
(Gasteropoda, Pulmonata, and Opisthobranchiata) are conveniently
divided into two sections, according as (1) there are separate
orifices for the male and female organs, or (2) one orifice serves for
both. To the former section (_Digonopora_[2]) belong the Limnaeidae,
Vaginulidae, and Onchidiidae, and many Opisthobranchiata, including
all the Pteropoda; to the latter (_Monogonopora_[255]) nearly all the
Nudibranchiate Opisthobranchiata, and all the rest of the Pulmonata.
In the latter case during union, mutual impregnation takes place, and
each of the two individuals concerned has been observed (compare p.
42) to deposit eggs. In the former, however, no such reciprocal act
can take place, but the same individual can play the part of male to
one and female to another, and we sometimes find a string of _Limnaea_
thus united, each being at once male and female to its two adjacent
neighbours.
=The Reproductive System.=--Broadly speaking, the complicated
arrangements which are found in Mollusca resolve themselves into
modifications of three important factors:--
(_a_) The _gonads_ or _germ-glands_, in which are developed the ova
and the spermatozoa. These glands are generally known as the _ovary_ in
the female, the sperm-gland or _testis_ in the male.
(_b_) The channels which provide for the passage of the seminal
products; namely, the _oviduct_ in the female, the _vas deferens_ or
sperm-duct in the male.
(_c_) The external generative organs.
[Illustration: FIG. 49.--Generative and other organs of
_Littorina obtusata_ L., _female_.
=A=, anus.
=Br=, branchia.
=Buc=, buccal mass.
=H=, heart.
=Hep=, hepatic duct.
=I=, continuation of oesophagus.
=Ki=, kidney.
=Li=, liver.
=M=, muscle of attachment.
=O´=, female orifice.
=Od=, oviduct.
=Oes=, oesophagus.
=Ov=, ovary.
=Ra=, radula.
=St=, stomach.
=U=, uterus.
(After Souleyet.)]
[Illustration: FIG. 50.--Generative and other organs of
_Littorina obtusata_ L., _male_.
=A=, anus.
=Br=, branchia.
=H=, heart.
=I=, intestine.
=Li=, liver.
=M=, muscle of attachment.
=Pe=, penis.
=Te=, testis.
=VD=, vas deferens.
(After Souleyet.)]
=Dioecious Mollusca.=--The common _Littorina obtusata_ will serve as
a typical instance of a dioecious prosobranchiate, exhibiting the
simplest form of organs. In the female the _ovary_, a lobe-shaped
body, is embedded in the liver. An _oviduct_ with many convolutions
conveys the ova into the _uterus_, an oblong chamber which consists
simply of a dilatation of the oviduct. The ova descend into the uterus,
which is sometimes furnished with a seminal pouch. In this seminal
pouch, or above it, in the oviduct, the ova come into contact with
the spermatozoa. The lower part of the uterus secretes a gelatinous
medium (or capsule, as the case may be) in which the fertilised ova
become enclosed previous to exclusion. In position the oviduct abuts
on the kidney, while the uterus is in close proximity to the rectum,
and the female external orifice is found close to the anus, within the
branchial cavity.
The male organs of _Littorina_ are more simple. The _testis_ is lodged,
like the ovary, in the liver; the _vas deferens_ is, like the oviduct,
convoluted, and eventually traverses the right side of the neck,
emerging near the right tentacle, and terminating in the _penis_ or
external copulative organ (Fig. 50).
This system prevails, with but slight modifications in detail,
throughout the prosobranchiate Gasteropoda. The most important
modification is the passage of the seminal products in certain cases
(many of the Diotocardia) through the right kidney, with which the
oviduct and vas deferens always stand in close relation. The same
arrangement occurs in the Scaphoda and some Pelecypoda.
The penis varies greatly in form and size. In the Strombidae (see
Fig. 99) and Buccinidae (Fig. 62) it is very large and prominent; in
_Littorina_ it is somewhat spinulose at one side; in _Paludina_ a
portion of it is lodged in the right tentacle, which becomes atrophied
and much more obtuse than the tentacle on the left side.
=Spermatozoa.=--The shape of the spermatozoa and of the ova in
Mollusca is of the usual type. In _Paludina Ampullaria_, and certain
species of _Murex_ two types of spermatozoa occur, one hair-like,
the other worm-like, three times as long as the former, and not
tapering at one end. The former type alone take part in fertilisation,
and penetrate the ovum. It has been suggested that these worm-like
spermatozoa are a kind of incipient ova, and indicate a possible
stage in commencing hermaphroditism. And, since the nearest allies of
the Prosobranchiata (in which these types occur) are hermaphrodite
(_i.e._ the Opisthobranchiata and Pulmonata), it is not unreasonable
to suppose that the Prosobranchiata should show some tendency towards
hermaphroditism in their genital glands.[256]
=Cephalopoda.=--The special characteristic of the reproductive organs
in female Cephalopoda is the development of various glands, some of
considerable size, in connexion with the ovary and oviduct. _Sepia_,
_Loligo_, and _Sepiola_ are furnished with two large _nidamental
glands_, which open into the mantle cavity independently of the
oviduct. Their purpose is to produce a viscid mucus, which envelops the
ova at the moment of their emission and eventually hardens into the
egg-capsules. A pair of accessory nidamental glands occur in _Sepia_,
as well as a pair of smaller glands situated on the oviduct itself.
In many of the male Cephalopoda the vas deferens is long and dilated
at its outer end into a glandular reservoir, within which are formed
the spermatophores, or narrow cylindrical packets which contain a very
large number of spermatozoa. When charged, the spermatophores pass
into what is known as Needham’s sac, where they remain until required
for use. These spermatophores are a very characteristic part of the
reproductive arrangements in the Cephalopoda. The male of _Sepia_ has
been noticed to deposit them, during union, upon the buccal membrane
of the female. During the emission of the ova by the female, the
spermatophores, apparently through the agency of a kind of spring
contained at one end, burst, and scatter the spermatozoa over the ova.
=The Hectocotylus Arm.=--Perhaps the most remarkable feature in the
sexual relations of all the Mollusca is the so-called _hectocotylus_
of the Cephalopoda. In the great majority of the male Cephalopoda, one
of the ‘arms,’ which is modified for the purpose in various ways and
to a greater or less extent, becomes charged with spermatophores, and
sometimes, during union, becomes detached and remains within the mantle
of the female, preserving for some considerable time its power of
movement.
The hectocotylus is confined to the dibranchiate Cephalopoda, and
its typical form, _i.e._ when part of the arm becomes disengaged and
left with the female, occurs only in three genera of the Octopodidae,
_viz._[_Argonauta_, _Ocythoe_ (_Philonexis_), and _Tremoctopus_. In
all of these, the male is many sizes smaller than the female. In
_Argonauta_ the third arm on the left side becomes hectocotylised. At
first it is entirely enveloped in a kind of cyst, in such a way that
only a small portion of the tip projects; subsequently the cyst parts
asunder, and allows the arm to become expanded to its full length,
which considerably exceeds that of the other arms. At a certain point
the acetabula or suckers terminate, and the remainder of the arm
consists of a very long, tapering, sometimes thread-like filament,
which is pointed at the extreme tip. It is not yet known how the
spermatophores find their way into the hectocotylus, or how the
hectocotylus impregnates the ova of the female. The arm thus affected
is not always the same. In _Tremoctopus_ it is the third of the right
side, in the _Decapoda_ the modification usually affects the fourth of
the left.
[Illustration: FIG. 51.--Male of _Ocythoe tuberculata_ Raf. (=
_Philonexis catenulatus_ Fér.), Mediterranean, showing three
stages, =A=, =B=, and =C=, in the development of the hectocotylus
arm: _h.cy_, hectocotylus still in the cyst; _c´y´_, spoon-shaped
cyst at the end of the arm when freed; _th_, thread-like organ
freed by the rupture of _c´y´_. Natural size. From specimens in
the British Museum.]
This singular property of the male Cephalopoda has only recently
been satisfactorily explained. It is true that Aristotle, more than
twenty-two centuries ago, distinctly stated that certain of the arms
were modified for sexual purposes. Speaking of what he calls the
polypus (which appears to represent the _Octopus vulgaris_ of the
Mediterranean), he says: ‘It differs from the female in having what the
fishermen call the white sexual organ _on its arm_;’ again, ‘Some say
that the male has something of a sexual nature (αὶδοιῶδές τι) on one of
its arms, that on which the largest suckers occur; that this is a kind
of muscular appendage attached to the middle of the arm, and that it is
entirely introduced within the funnel of the female’. Unfortunately the
word translated by _introduced_ is corrupt, and can only be restored
conjecturally. He again remarks, ‘The last of the arms, which tapers to
a fine point and is the only whitish arm, it uses in sexual union.’[257]
The typical hectocotylus seems to have entirely escaped notice until
early in the present century, when both Delle Chiaje and Cuvier
described it, as detected within the female, as a _parasite_, the
latter under the name of _Hectocotylus octopodis_. Kölliker, in
1845–49, regarded the Hectocotylus of _Tremoctopus_ as the entire male
animal, and went so far as to discern in it an intestine, heart, and
reproductive system. It was not until 1851 that the investigations of
Vérany and Filippi confirmed a suggestion of Dujardin,[258] while H.
Müller, in 1853, completed the discovery by describing the entire male
of _Argonauta_.
In all genera of dibranchiate Cephalopoda except _Argonauta_,
_Ocythoe_, and _Tremoctopus_, one of the arms is sexually modified
in various ways, but never becomes so much prolonged, and is never
detached and left with the female. In _Loligo Forbesii_ Stp. the fourth
arm on the left has 23 pairs of regularly developed acetabula, which
then lessen in size and disappear, being replaced by long pedunculated
papillae, of which there are about 40 pairs. In _Loligo vulgaris_ Lam.
and _L. Pleii_ Orb. 18 or 19 pairs of acetabula are regularly formed,
and then occur 40 pairs of papillae, as in _Forbesii_. In other species
of _Loligo_ (_gahi_ Orb., _brevis_ Bl., _brasiliensis_ Orb.) only the
outer row of suckers becomes modified into papillae after about the
20th to the 22nd pair. In _Sepioteuthis sepioides_ the modification
is the same as in the _Loligo_ last mentioned, but the corresponding
arm on the right side is so covered with acetabula towards its extreme
end, that it is thought that it in some way co-operates with the
hectocotylised left arm.
In _Octopus_, the third arm on the right side is subject to
modification. This arm is always shorter than the corresponding arm on
the other side, and carries fewer suckers, but is furnished at the
extreme tip with a peculiar kind of plate, which connects with the
membrane at the base of the arm by a channel of skin, which probably
conveys the spermatophores up to the tip.
In _Octopus vulgaris_, the species referred to by Aristotle, the
hectocotylised arm is short, thin in its outer half and pointed at the
extremity, while the fold of skin is very white, and gives the arm
an appearance of being divided by a cleft at the side. At the same
time, an unusual development of one or two suckers on the arm is not
uncommon.[259]
[Illustration: FIG. 52.--_Octopus lentus_ Baird, N. Atlantic,
showing the peculiar formation of the hectocotylus arm, _h.a_.
(After Verrill, × ½.)]
It is believed that in the Tetrabranchiate Cephalopoda (_Nautilus_) a
union of the four inner ventral arms may correspond functionally to the
hectocotylising of the arm in the Dibranchiates.
=Hermaphrodite Mollusca.=--(_a_) _Monogonopora._--The reproductive
system in the hermaphrodite Mollusca is far more complicated than in
the dioecious, from the union of the male and female organs in the same
individual. As a type of the Monogonopora, in which a single orifice
serves for both male and female organs, may be taken the common garden
snail (_Helix aspersa_), the accompanying figure of which is drawn from
two specimens found in the act of union (Fig. 53).
[Illustration: FIG. 53.--Genitalia of _Helix aspersa_ Müller,
drawn from two individuals in the act of union, from a dissection
by F. B. Stead.
=A.G=, albumen gland.
=C=, coecum.
=Cr=, crop.
=D.S=, dart sac.
=E=, eye (retracted).
=Fl=, flagellum.
=H.D=, hermaphrodite duct.
=H.DF=, ditto, female portion.
=H.DM=, ditto, male portion.
=H.G=, hermaphrodite gland.
=L=, liver.
=M.G=, =M.G=, mucous glands.
=Ov=, oviduct.
=P.S=, penis sac.
=R.M=, retractor muscle of penis.
=Sp=, spermatheca.
=V=, vagina.
=V.D=, vas deferens.]
Beginning from the inside and proceeding outwards we have firstly the
_hermaphrodite gland_ or _ovo-testis_ (H.G.), a yellowish white mass
of irregular shape, embedded in the _liver_ (L.) and forming part
of its spiral but not reaching quite to the apex. Within this gland
are developed the ova and spermatozoa. The former are rather large
round cells, produced within the outer wall of the gland, while
the spermatozoa, which are produced in the more central part, are
thread-like bodies, generally aggregated in small bundles. From the
hermaphrodite gland the ova and spermatozoa pass through the upper
part of the _hermaphrodite duct_ (H.D.), which is always more or less
convoluted. Below the convoluted portion, the duct opens into the
_albumen gland_ (A.G.), a large linguiform mass of tissue which becomes
dilated at the time of pairing, and secretes a thick viscid fluid which
probably serves to envelop the ova. Up to this point both the male and
female elements follow the same course, but on their exit from the
albumen gland they diverge. The hermaphrodite duct becomes greatly
enlarged, and is partially divided by a kind of septum into a male and
female portion. These run parallel to one another, the larger or female
portion (H.DF.), through which the ova pass (and which is sometimes
termed the _uterus_) being dilated into a number of puckered folds,
while the smaller or male portion (H.DM.) is comparatively narrow,
and not dilated. At their anterior end, the two portions of the duct
separate completely from one another, the female portion being then
termed the _oviduct_ (OV.) and the male portion the _vas deferens_
(V.D.).
Following first the oviduct, we find that it soon widens into the
_vagina_ (V.), which is furnished with a pair of _mucous glands_
(M.G.), one on each side. These are much branched, and resemble little
bunches of whitish seaweed. A little above the mucous glands a long
tube diverges from the vagina, which is furnished with a produced
_coecum_ (C.) and a pouch, the _spermatheca_ (SP.) at the extreme
end. In this pouch, and in the duct leading to it, is stored the
spermatophore received in union with another snail. Just below the
mucous glands the vagina is joined by the _dart sac_ (D.S.), which is
more fully described below. Finally, at its lower end the vagina unites
with the penis sac at a point just posterior to the common orifice.
Returning now to the male organs, we find that the _vas deferens_
is the continuation of the male portion of the hermaphrodite duct,
after its final separation from the female portion. It passes under
the retractor muscle of the upper right tentacle, which has been cut
away in the specimen figured, to dissect it out. Just before the vas
deferens widens into the penis sac, it branches off into a long and
tapering tube, the _flagellum_, in which the spermatozoa are stored and
become massed together in the long packet known as the _spermatophore_.
The _penis sac_ (P.S.) is the continuation of the vas deferens beyond
the point at which the flagellum diverges. It joins the vagina at
its extreme anterior end, uniting with it to form the common genital
aperture, which cannot be exactly represented in the figure. The
_penis_ itself lies in the interior of the penis sac, and is a rather
long muscular tube which is protruded during union, but at other times
remains retracted within the sac.
In the Helicidae generally, the form of the generative organs varies
with each separate species, sometimes merely as regards the size of
the different parts, at others in the direction of greater simplicity
or complication. The mucous glands may be absent, and the flagellum
greatly reduced in size, or absent altogether.
=The Dart Sac.=--A remarkable part of the reproductive system in many
of the true Helicidae is the so-called _dart_, _Liebespfeil_, or _telum
veneris_. It consists of ‘a straight, or curved, sometimes slightly
twisted tubular shaft of carbonate of lime, tapering to a fine point
above, and enlarging gradually, more often somewhat abruptly, to
the base.’ The sides of the shaft are sometimes furnished with two
or more blades; these are apparently not for cutting purposes, but
simply to brace the stem. The dart is contained in a dart sac, which
is attached as a sort of pocket to the vagina, at no great distance
from its orifice. There are four different forms of sac. It may be
single or double, and each of these divisions may be bilobed, each lobe
containing one dart at a time. In _Helix aspersa_ the dart is about
5/16 in. in length, and ⅛ in. in breadth at its base (see Fig. 54).
It appears most probable that the dart is employed as an adjunct to
the sexual act. Besides the fact of the position of the dart sac
anatomically, we find that the darts are extruded and become embedded
in the flesh just before or during the act of copulation. It may
be regarded, then, as an organ whose punctures induce excitement
preparatory to sexual union. It only occurs in well-grown specimens.
When once it begins to form, it grows very rapidly, perhaps not more
than a week being required for its entire formation.
[Illustration: FIG. 54.--Darts of British land snails: =A=,
_Hyalinia excavata_ Bean; =B=, _Helix hortensis_ Müll.; =C=,
_Helix aspersa_ Müll. (After Ashford.)]
The dart is almost confined to _Helicidae_, a certain number of
exceptions being known which border on _Helix_. _Hyalinia nitida_ and
_excavata_ are the only British species, not _Helices_, which are known
to possess it. It has not been noticed to occur in the slugs, except in
the N. American genus _Tebennophorus_. About one-third of the British
Helices are destitute of the dart.[260] _H. rufescens_ possesses a
double bilobed sac, but only two darts, which lie in the lower lobes.
It does not use the darts, and could not do so, from the relative sizes
of dart and sac; it has often been watched when uniting, but the use of
the darts has never been observed. From this it has been inferred that
the darts are degenerate weapons of defence, and that they were in fact
at one time much stronger organs and more often used.[261] This theory,
however, does not seem consistent with the whole circumstances of the
occurrence, position, and present use of the darts.