the _Digonopora_ or hermaphrodite Mollusca with separate generative
apertures for the male and female organs, we may take the common
_Limnaea stagnalis_ (Fig. 55). It will be seen from the figure that
the relative positions of the hermaphrodite gland and duct, and of
the albumen gland, are the same as in _Helix_. When the oviduct parts
company from the vas deferens, it becomes furnished with several
accessory glands, one of which (GL.E.) probably serves as a reservoir
for the ova, and answers more or less to a uterus. The tube leading to
the spermatheca is short, and there is no divergent caecum. The female
orifice lies near to the external opening of the branchial cavity. The
vas deferens, which is very long, is furnished with a large prostate
gland. The penis sac is greatly dilated, and there is no flagellum. The
male orifice is behind the right tentacle, slightly in advance of the
female orifice (compare Fig. 102).
[Illustration: FIG. 55.--Genitalia of _Limnaea stagnalis_ L.
(from a dissection by F. B. Stead), × 2.
=A.G=, albumen gland.
=Ac.G=, accessory gland.
=F.O=, female orifice.
=Gl.E=, glandular enlargement.
=H.D=, hermaphrodite duct.
=H.G=, hermaphrodite gland.
=Li=, liver.
=M.O=, male orifice.
=P=, penis sac.
=Pr=, prostate.
=R.M=, retractor muscle of penis.
=Sp=, spermatheca.
=V.D=, vas deferens.]
Most of the _Opisthobranchiata_, but not all, have separate sexual
orifices. Numerous variations from the type just described will be
found to occur, particularly in the direction of the development of
accessory glands, which are sometimes very large, and whose precise
purpose has in many cases not been satisfactorily determined.
_Pelecypoda._--In the dioecious Pelecypoda, which form the great
majority, the reproductive system is simple, and closely parallel in
both sexes. It consists of a pair of gonads, which are either ovaries
or testes, and a pair of oviducts or sperm-ducts which lead to a
genital aperture. The gonads are usually placed symmetrically at the
sides or base of the visceral mass. The oviduct is short, and the
genital aperture is usually within the branchial chamber, thus securing
the fertilisation of the ova by the spermatozoa, which are carried into
the branchial chamber with the water which passes through the afferent
siphon.
Hermaphrodite Pelecypoda are rare, the sexes being usually separate.
The following are assured instances: _Pecten glaber_, _P. jacobaeus_,
_P. maximus_, _Ostrea edulis_, _Cardium norvegicum_, _Pisidium
pusillum_, _Cyclas cornea_, _Pandora rostrata_, _Aspergillum
dichotomum_, and perhaps _Clavagella_. The greater number of these
have only a single genital gland (gonad) on each side, with a single
efferent duct from each, but part of the gland is male and part female,
_e.g._ in the Pectens above mentioned. _Pandora_ and _Aspergillum_ have
two distinct glands, respectively male and female, on each side, each
of the two glands possessing its separate duct, and the two ducts from
each side eventually opening near one another. It appears probable that
the _Septibranchiata_ (_Cuspidaria_, _Poromya_, _Lyonsiella_, etc.)
must also be added to the number of hermaphrodite Pelecypoda which have
separate male and female glands.
It is worthy of remark that all the hermaphrodite Pelecypoda belong to
forms decidedly specialised, while forms distinctly primitive, such as
_Nucula_, _Solenomya_, _Arca_, and _Trigonia_ are all dioecious. In
Gasteropoda similarly, the least specialised forms (the _Amphineura_,
with the exception of the _Neomeniidae_, and the _Rhipidoglossa_)
are dioecious. It is possible therefore that in the ancestors of the
Mollusca the separation of the sexes had already become the normal
type of things, and that hermaphroditism in the group is, to a certain
extent, a sign or accompaniment of specialisation.[262]
=Development of Fresh-water Bivalves.=--The vast majority of
fresh-water bivalves either pass the larval stage entirely within
the mother, and do not quit her except in a perfectly developed form
(_Cyclas_, _Pisidium_), or assume a mode of development in which free
larvae indeed occur, but are specially modified for adaptation to
special circumstances (_Unio_). _Cyclas_ and _Pisidium_, and no doubt
all the kindred genera, preserve their ova in a sort of brood-pouch
within the gills, in which the ova pass the earlier stages of their
development. But, even so, the larva of these genera retains some
traces of its original free-swimming habits, for a rudimentary velum,
which is quite useless for its present form of development, has been
detected in _Cyclas_.
The larva of _Dreissensia_ (see Fig. 47, A), so far as is at present
known, stands alone among fresh-water bivalves in being free-swimming,
and to this property has been attributed, no doubt with perfect
justice, the fact of the extraordinarily rapid spread of _Dreissensia_
over the continent of Europe (chap. xvi.). In expelling the ova,
the parent slightly opens the shells and then quickly closes them,
shooting out a small point of white slime, which is in fact a little
ball of eggs. The general course of development is precisely parallel
to that of marine _Pelecypoda_, greatly resembling, so far as form is
concerned, certain stages in the growth of the larvae of _Modiolaria_
and _Cardium_, as figured by Lovén.[263]
In June and July the larvae appear in large numbers on the surface of
the water, when in spite of their exceedingly small size, they can
be captured with a fine hand-net. They pass about eight days on the
surface, feeding apparently on minute floating algae. During this time,
the principal change they undergo is in the formation of the foot,
which first appears as a small prominence midway between the mouth
and anus, and gradually increases in length and flexibility. When the
larva sinks to the bottom, the velum soon disappears entirely, the
foot becomes exceedingly long and narrow, while the shell is circular,
strongly resembling a very young _Cyclas_.
=Larvae of Unionidae.=--The early stages of the development of _Unio_
and _Anodonta_ (so far as the species of North America, Europe, and
Asia are concerned) is of extreme interest, from the remarkable fact
that the young live for some time parasitically attached to certain
species of fresh-water fishes. In order to secure this attachment,
the larva, which is generally known as _Glochidium_, develops a long
filament which perhaps renders it aware of the neighbourhood of a fish,
and also a larval shell furnished with strong hooks by which it fastens
itself to the body of its unconscious host (Fig. 56). According to some
interesting observations made by Mr. O. H. Latter,[264] the ova pass
into the external gill of the mother, in which is secreted a nutritive
mucus on which they are sustained until they arrive at maturity and a
suitable opportunity occurs for their ‘being born.’ If this opportunity
is deferred, and the _Glochidia_ mature, their so-called ‘byssus’
becomes developed, and by being entangled in the gill filaments of the
parent, prevents their escaping. It is interesting to notice that, when
the nutritive mucus of the parent is used up, it becomes, as it were,
the turn of the children to provide for themselves a secondary mode of
attachment.
[Illustration: FIG. 56.--=A=, _Glochidium_ immediately after it
is hatched: _ad_, adductor muscle; _by_, ‘byssus’ cord; _s_,
sense organs; _sh_, shell. =B=, _Glochidium_ after it has been on
the fish for some weeks: _a.ad_, _p.ad_, anterior and posterior
adductors; _al_, alimentary canal; _au.v_, auditory vesicle;
_br_, branchiae; _f_, foot; _mt_, mantle. (Balfour.)]
The mother _Anodonta_ does not always retain the _Glochidium_ until
fish are in her neighbourhood. Gentle stirring of the water caused
them to emit _Glochidium_ in large masses, if the movement was not so
violent as to cause alarm. The long slimy masses of _Glochidium_ were
observed to be drawn back again within the shell of the mother, even
after they had been ejected to a distance of 2 or 3 inches.
It is a mistake to assert that the young _Glochidium_ can swim. When
they finally quit the mother, they sink to the bottom, and there
remain resting on their dorsal side, with the valves gaping upwards and
the so-called byssus streaming up into the water above them. There they
remain, until a convenient ‘host’ comes within reach, and if no ‘host’
comes within a certain time, they perish. They are evidently peculiarly
sensitive to the presence of fish, but whether they perceive them by
smell or some other sense is unknown. “The tail of a recently killed
stickleback thrust into a watch-glass containing _Glochidium_ throws
them all into the wildest agitation for a few seconds; the valves are
violently closed and again opened with astonishing rapidity for 15–25
seconds, and then the animals appear exhausted and lie placid with
widely gaping shells--unless they chance to have closed upon any object
in the water (_e.g._ another _Glochidium_), in which case the valves
remain firmly closed.”
In about four weeks after the _Glochidium_ has quitted its host, and
the permanent shell has made its appearance _within_ the two valves
of the _Glochidium_, the projecting teeth of the latter press upon
the ventral edge of the permanent shell, at a point about half way in
its lengthward measurement, retarding the growth of the shell at that
particular point, and indenting its otherwise uninterrupted curve with
an irregular notch or dent. As growth proceeds, this dent becomes less
and less perceptible on the ventral margin of the shell itself, but its
effects may be detected, in well-preserved specimens, by the wavy turn
in the lines of growth, especially near the umbones of the young shell.
Mr. Latter found that all species of fish with which he experimented
had a strong dislike to _Glochidium_ as an article of food. Sometimes
a fish would taste it “just to try,” but invariably spit it out again
in a very decided manner. The cause of unpleasantness seemed not to be
the irritation produced in the mouth of the fish by the attempt of the
_Glochidium_ to attach itself, but was more probably due to what the
fish considered a nasty taste or odour in the object of his attentions.
* * * * *
The following works will be found useful for further study of this
portion of the subject:--
=F. M. Balfour=, Comparative Embryology, vol. i. pp. 186–241.
=F. Blochmann=, Ueber die Entwickelung von Neritina fluviatilis
Müll.: Zeit. wiss. Zool. xxxvi. (1881), pp. 125–174.
=L. Boutan=, Recherches sur l’anatomie et le développement de la
Fissurelle: Arch. Zool. exp. gén. (2) iii. suppl. (1885), 173
pp.
=W. K. Brooks=, The development of the Squid (Loligo Pealii
Les.): Anniv. Mem. Bost. Soc. Nat. Hist. 1880.
„ „ The development of the oyster: Studies Biol. Lab. Johns
Hopk. Univ. i. (1880), 80 pp.
=R. von Erlanger=, Zur Entwickelung von Paludina vivipara:
Morph. Jahrb. xvii. (1891), pp. 337–379, 636–680.
„ „ Zur Entwickelung von Bythinia tentaculata: Mitth. Zool.
Stat. Neap, x. (1892), pp. 376–406.
=H. Fol=, Sur le développement des Ptéropodes: Arch. Zool. exp.
gén. iv. (1875), pp. 1–214.
„ Etudes sur le développement des Mollusques. Hétéropodes: ibid
v. (1876), pp. 105–158.
„ Etudes sur le développement des Gastéropodes pulmonés: ibid.
viii. (1880), pp. 103–232.
=H. Grenacher=, Zur Entwickelungsgeschichte der Cephalopoden:
Zeit. wiss. Zool. xxiv. (1874), pp. 419–498.
=B. Hatschek=, Ueber Entwickelungsgeschichte von Teredo: Arb.
Zool. Inst. Univ. Wien, iii. (1881), pp. 1–44.
=R. Horst=, On the development of the European oyster: Quart.
Journ. Micr. Sc. xxii. (1882), pp. 339–346.
=E. Korschelt= and =K. Heider=, Lehrbuch der vergleichenden
Entwickelungsgeschichte der wirbellosen Thiere, Heft iii.
(1893), pp. 909–1177 (the work is in process of translation into
English).
=A. Kowalewsky=, Embryogénie du Chiton polii avec quelques
remarques sur le développement des autres Chitons: Ann. Mus.
Hist. Nat. Mars. Zool. i. (1883), v.
=E. Ray Lankester=, Contributions to the developmental history
of the Mollusca: Phil. Trans. Roy. Soc. vol. 165 (1875), pp.
1–31.
„ „ Observations on the development of the pond-snail
(Lymnaeus stagnalis), and on the early stages of other Mollusca:
Quart. Journ. Micr. Sc. xiv. (1874), pp. 365–391.
„ „ Observations on the development of the Cephalopoda: ibid.
xv. (1875), pp. 37–47.
=W. Patten=, The embryology of Patella: Arb. Zool. Inst. Univ.
Wien, vi. (1886), pp. 149–174.
=M. Salensky=, Études sur le développement du Vermet: Arch.
Biol. vi. (1885), pp. 655–759.
=L. Vialleton=, Recherches sur les premières phases du
développement de la Seiche (Sepia officinalis): Ann. Sc. Nat.
Zool. (7) vi. (1888), pp. 165–280.
=S. Watase=, Observations on the development of Cephalopods:
Stud. Biol. Lab. Johns Hopk. Univ. iv. (1888), pp. 163–183.
„ „ Studies on Cephalopods: Journ. Morph. iv. (1891), pp.
247–294.
=E. Ziegler=, Die Entwickelung von Cyclas cornea Lam.: Zeit.
wiss. Zool. xli. (1885), pp. 525–569.
CHAPTER VI
RESPIRATION AND CIRCULATION--THE MANTLE
The principle of respiration is the same in the Mollusca as in all
other animals. The blood is purified by being brought, in successive
instalments, into contact with pure air or pure water, the effect of
which is to expel the carbonic acid produced by animal combustion, and
to take up fresh supplies of oxygen. Whether the medium in which a
mollusc lives be water or air, the effect of the respiratory action is
practically the same.
Broadly speaking, Mollusca whose usual habitat is the water ‘breathe’
water, while those whose usual habitat is the land ‘breathe’ air. But
this rule has its exceptions on both sides. The great majority of the
fresh-water Mollusca which are not provided with an operculum (_e.g._
_Limnaea_, _Physa_, _Planorbis_), breathe air, in spite of living in
the water. They make periodic visits to the surface, and take down
a bubble of air, returning again for another when it is exhausted.
On the other hand many marine Mollusca which live between tide-marks
(_e.g._ _Patella_, _Littorina_, _Purpura_, many species of _Cerithium_,
_Planaxis_, and _Nerita_) are left out of the water, through the
bi-diurnal recess of the tide, for many hours together. Such species
invariably retain several drops of water in their branchiae, and,
aided by the moisture of the air, contrive to support life until the
water returns to them. Some species of _Littorina_ (_e.g._ our own _L.
rudis_ and many tropical species) live so near high-water mark that
at neap-tides it must frequently happen that they are untouched by
the sea for several weeks together, while they are frequently exposed
to a burning sun, which beats upon the rocks to which they cling. In
this case it appears that the respiratory organs will perform their
functions if they can manage to retain an extremely small amount of
moisture.[265]
The important part which the respiratory organs play in the economy of
the Mollusca may be judged from the fact that the primary subdivision
of the Cephalopoda into Dibranchiata and Tetrabranchiata is based
upon the number of branchiae they possess. Further, the three great
divisions of the Gasteropoda have been named from the position or
character of the breathing apparatus, _viz_. Prosobranchiata,
Opisthobranchiata and Pulmonata, while the name Pelecypoda has hardly
yet dispossessed Lamellibranchiata, the more familiar name of the
bivalves.
Respiration may be conducted by means of--(_a_) Branchiae or Gills,
(_b_) a Lung or Lung-cavity, (_c_) the outer skin.
In the Pelecypoda, Cephalopoda, Scaphopoda, and the great majority of
the Gasteropoda, respiration is by means of branchiae, also known as
_ctenidia_[266], when they represent the primitive Molluscan gill and
are not ‘secondary’ branchiae (pp. 156, 159).
In all non-operculate land and fresh-water Mollusca, in the
Auriculidae, and in one aberrant operculate (_Amphibola_), respiration
is conducted by means of a lung-cavity, or rarely by a true lung,
whence the name _Pulmonata_. The land operculates (Cyclophoridae,
Cyclostomatidae, Aciculidae, and Helicinidae) also breathe air, but are
not classified as Pulmonata, since other points in their organisation
relate them more closely to the marine Prosobranchiata. Both methods of
respiration are united in _Ampullaria_, which breathes indifferently
air through a long siphon which it can elevate above the surface of the
water, and water through a branchia (see p. 158). _Siphonaria_ (Fig.
57) is also furnished with a lung-cavity as well as a branchia. Both
these genera may be regarded as in process of change from an aqueous
to a terrestrial life, and in _Siphonaria_ the branchia is to a great
extent atrophied, since the animal is out of the water, on the average,
twenty-two hours out of the twenty-four. In the allied genus _Gadinia_,
where there is no trace of a branchia, but only a lung-cavity, and
in _Cerithidea obtusa_, which has a pulmonary organisation exactly
analogous to that of _Cyclophorus_,[267] this process may be regarded
as practically completed.
[Illustration: FIG. 57.--=A=, _Siphonaria gigas_ Sowb., Panama,
the animal contracted in spirit: _gr_, siphonal groove on right
side. =B=, _Gadinia peruviana_, Sowb., Chili, shell only: _gr_,
mark of siphonal groove to right of head.]
Respiration by means of the skin, without the development of any
special organ, is the simplest method of breathing which occurs in the
Mollusca. In certain cases, _e.g._ _Elysia_, _Limapontia_, and _Cenia_
among the Nudibranchs, and the parasitic _Entoconcha_ and _Entocolax_,
none of which possess breathing organs of any kind, the whole outer
surface of the body appears to perform respiratory functions. In
others, the dorsal surface is covered with papillae of varied size and
number, which communicate with the heart by an elaborate system of
veins. This is the case with the greater number of the _Aeolididae_
(Fig. 58, compare Fig. 5, C), but it is curious that when the animal is
entirely deprived of these papillae, respiration appears to be carried
on without interruption through the skin.
[Illustration: FIG. 58.--_Aeolis despecta_ Johnst., British
coasts. (After Alder and Hancock.)]
In the development of a distinct breathing organ, it would seem as if
progress had been made along two definite lines, each resulting in
the exposure of a larger length of veins, _i.e._ of a larger amount
of blood, to the simultaneous operation of fresh air or fresh water.
Either (_a_) the skin itself may have developed, at more or less
regular intervals, elevations, or folds, which gradually took the form
of papillae, or else (_b_) an inward folding, or ‘invagination,’ of
the skin, or such a modification of the mantle-fold as is described
below (p. 172) may have taken place, resulting in the formation of a
cavity more or less surrounded by walls, within which the breathing
organs were ultimately developed. Sometimes a combination of both
processes seems to have occurred, and after a papilliform organ has
been produced, an extension or prolongation of the skin has taken
place, in order to afford a protection to it. Respiration by means of a
lung-cavity is certainly subsequent, in point of time, to respiration
by means of branchiae.
[Illustration: FIG. 59.--_Chiton squamosus_ L., Bermuda: =A=,
anus; =Br=, branchiae; =M=, mouth.]
[Illustration: FIG. 60.--_Fissurella virescens_ Sowb., Panama,
showing position of the branchiae: =Br=, branchiae: =E=, =E=,
eyes; =F=, foot; =M=, mantle; =T=, =T=, tentacles.]
The branchiae seem to have been originally paired, and arranged
symmetrically on opposite sides of the body. It is not easy to decide
whether the multiple form of branchia which occurs in _Chiton_ (Fig.
59), or the simple form as in _Fissurella_ (Fig. 60), is the more
primitive. Some authorities hold that the multiple branchia has
gradually coalesced into the simple, others that the simple form has
grown, by serial repetition, into the multiple. There appears to be no
trace of any intermediate forms, and, as a matter of fact, the multiple
branchia is found only in the _Amphineura_, while one or rarely
two (never more) pairs of branchiae, occur, with various important
modifications, in the vast majority of the Mollusca.
_Amphineura._--In _Chiton_ the branchiae are external, forming a long
row of short plumes, placed symmetrically along each side of the foot.
The number of plumes, at the base of each of which lies an osphradial
patch, varies from about 70 to as few as 6 or 7. When the plumes are
few, they are confined to the posterior end, and thus approximate
to the form and position of the branchiae in the other Amphineura.
In _Chaetoderma_, the branchiae consist of two small feather-shaped
bodies, placed symmetrically on either side of the anus, which opens
into a sort of cloaca within which the branchiae are situated. In
_Neomenia_ the branchiae are still further degraded, consisting
of a single bunch of filaments lying within the cloaca, while in
_Proneomenia_ there is no more than a few irregular folds on the
cloaca-wall (Fig. 61).
[Illustration: FIG. 61.--Terminal portions of the Amphineura,
illustrating the gradual degradation of the branchiae, and
their grouping round the anus in that class. =A=, _Chiton_
(_Hemiarthrum_) _setulosus_ Carp., Torres Str.; =B=, _Chiton_
(_Leptochiton_) _benthus_ Hadd., Torres Str.; =C=, _Chaetoderma_;
=D=, _Neomenia_; _a_, anus; _br_, _br_, branchiae; _k_, _k_,
kidneys; _p_, pericardium. (=A= and =B= after Haddon, =C= and =D=
after Hubrecht.)]
In the _Prosobranchiata_, symmetrically paired branchiae occur only
in the Fissurellidae, Haliotidae, and Pleurotomariidae, in the former
of which two perfectly equal branchiae are situated on either side
of the back of the neck. These three families taken together form
the group known as _Zygobranchiata_.[268] In all other families the
asymmetry of the body has probably caused one of the branchiae,
the right (originally left), to become aborted, and consequently
there is only one branchia, the left, in the vast majority of
marine Prosobranchiata, which have been accordingly grouped as
_Azygobranchiata_. Even in _Haliotis_ the right branchia is rather
smaller than the left, while the great size of the attachment muscle
causes the whole branchial cavity to become pushed over towards the
left side. In those forms which in other respects most nearly approach
the Zygobranchiata, namely, the Trochidae, Neritidae, and Turbinidae,
the branchia has two rows of filaments, one on each side of the long
axis, while in all other Prosobranchiata there is but one row (see Fig.
79, p. 169).
[Illustration: FIG. 62.--_Bullia laevissima_ Gmel., showing
branchial siphon =S=; =F=, =F=, =F=, foot; =OP=, operculum; =P=,
penis; =Pr=, proboscis; =T=, =T=, tentacles. (After Quoy and
Gaimard.)]
In the great majority of marine Prosobranchiata the branchia is
securely concealed within a chamber or pouch (the respiratory cavity),
which is placed on the left dorsal side of the animal, generally near
the back of the neck. For breathing purposes, water has to be conveyed
into this chamber, and again expelled after it has passed over the
branchia. In the majority of the vegetable-feeding molluscs (_e.g._
_Littorina_, _Cerithium_, _Trochus_) water is carried into the chamber
by a simple prolongation of one of the lobes or lappets of the mantle,
and makes its exit by the same way, the incoming and outgoing currents
being separated by a valve-like fringe depending from the lobe. In the
carnivorous molluscs, on the other hand, a regular tube, the _branchial
siphon_, which is more or less closed, has been developed from a fold
of the mantle surface, for the special purpose of conducting water to
the branchia. After performing its purpose there, the spent water does
not return through the siphon, but is conducted towards the anus by
vibratile cilia situated on the branchiae themselves. In a large number
of cases, this siphon is protected throughout its entire length by a
special prolongation of the shell called the canal. Sometimes, as in
_Buccinum_ and _Purpura_, this canal is little more than a mere notch
in the ‘mouth’ of the shell, but in many of the Muricidae (_e.g._ _M.
haustellum_, _tenuispina_, _tribulus_) the canal becomes several inches
long, and is set with formidable spines (see Fig. 164, p. 256). In
_Dolium_ and _Cassis_ the canal is very short, but the siphon is very
long, and is reflected back over the shell.
The presence or absence of this siphonal notch or canal forms a fairly
accurate indication of the carnivorous or vegetarian tendencies of most
marine Prosobranchiata, which have been, on this basis, subdivided
into _Siphonostomata_ and _Holostomata_. But this classification is
of no particular value, and is seriously weakened by the fact that
_Natica_, which is markedly ‘holostomatous,’ is very carnivorous, while
_Cerithium_, which has a distinct siphonal notch, is of vegetarian
tendencies.
In the Zygobranchiata the water, after having aerated the blood in the
branchiae, usually escapes by a special hole or holes in the shell,
situated either at the apex (_Fissurella_) or along the side of the
last whorl (_Haliotis_). In _Pleurotomaria_ the slit answers a similar
purpose, serving as a sluice for the ejection of the spent water, and
thus preventing the inward current from becoming polluted before it
reaches the branchiae (see Fig. 179, p. 266).
In _Patella_ the breathing arrangements are very remarkable. In spite
of their apparent external similarity, this genus possesses no such
symmetrically paired plume-shaped branchiae as _Fissurella_, but we
notice a circlet of gill-lamellae, which extends completely round the
edge of the mantle. It has been shown by various authorities that
these lamellae are in no sense morphologically related to the paired
branchiae in other Mollusca, but only correspond to them functionally.
The typical paired branchiae, as has been shown by Spengel, exist
in _Patella_ in a most rudimentary form, being reduced to a pair
of minute yellow bodies on the right and left sides of the back of
the ‘neck.’ A precisely similar abortion of the true branchiae, and
special development of a new organ to perform their work, is shown in
_Phyllidia_ and _Pleurophyllidia_ (see below under Opisthobranchiata).
This circlet of functional gills in _Patella_ has therefore little
systematic value, being only developed in an unusual position, like
the eyes on the mantle in certain _Pelecypoda_, to supply the place of
the true organs which have fallen into disuse. Accordingly Cuvier’s
class of _Cyclobranchiata_, which included _Patella_ and _Chiton_, has
no value, and has indeed long been discarded. In _Chiton_ the gills
never extend completely round the animal, but are always more or less
interrupted at the head and anus. They are the true gills, the plumes
being serially repeated in the same way as the shell plates.
[Illustration: FIG. 63.--_Patella vulgata_ L., seen from the
ventral side: _f_, foot; _g.l_, circlet of gill lamellae; _m.e_,
edge of the mantle; _mu_, attachment muscle; _sl_, slits in the
same; _sh_, shell; _v_, vessel carrying aerated blood to the
heart; _v´_, vessel carrying blood from the heart; _ve_, small
accessory vessels.]
[Illustration: FIG. 64.--_Patella vulgata_ L., seen from the
dorsal side after the removal of the shell and the black pigment
covering the integument; the anterior portion of the mantle is
cut away or turned back: _a_, anus; _br_, _br_, remains of the
true branchiae (ctenidia); _i_, intestine; _k_, _k´_, kidneys;
_k.ap_, their apertures on each side of the anus; _l_, liver;
_m_, _m_, mantle; _mu_, attachment muscles, severed in removal of
shell; _t_, _t_, tentacles.]
In the land Prosobranchiata (Cyclostomatidae, Cyclophoridae,
Aciculidae, Helicinidae) which, having exchanged a marine for an
aerial life, breathe air instead of water, the branchia has completely
disappeared, and breathing is conducted, as in the Pulmonata, by a
lung-cavity. In certain genera of land operculates, _e.g._ _Pupina_,
_Cataulus_, _Pterocyclus_, a slight fissure or tube in the last whorl
(see Fig. 180, p. 266) serves to introduce air into the shell, which
is perhaps otherwise closed to air by the operculum. In _Aulopoma_,
which has no tube, the operculum admits free circulation of air. In
certain other Cyclostomatidae the apex is truncated, and air can enter
there. De Folin closed with wax the aperture of _Cycl. elegans_, and
found that on placing it in a pneumatic machine, the shell gave off
air through its whole surface. On the other hand, _Cylindrella_ and
_Stenogyra decollata_, on being submitted to the same test, showed that
the truncated part alone was permeable by air.
Fischer and Bouvier have made some interesting observations on the
breathing of a species of _Ampullaria_ (_insularum_ Orb.). The species
has, in common with all _Ampullaria_, two siphons, but while the
right siphon is but slightly developed, the left is very long, almost
twice as long as the shell (see Fig. 65). The animal, when under the
water, lengthens its siphon, brings the orifice to the surface, and by
alternately raising and depressing its head produces in the pulmonary
sac movements of ex- and inspiration; these are repeated about ten or
fifteen times at regular intervals of from six to eight seconds, a
method of respiration strongly resembling that of the Cetacea. At the
same time, branchial respiration takes place. If powdered carmine is
added to water, the particles are seen to enter the branchial cavity
by the siphon and pass out by the short right siphon. Sometimes the
animal remains under water for hours without rising to the surface to
inspire air. In _Valvata_ (Fig. 66) the branchia is very large, and
projects like a leaf or fan above the shell on the left side; on the
corresponding position on the right side is a long filiform appendage,
which some have regarded as representing the other branchia.
[Illustration: FIG. 65.--_Ampullaria insularum_ Orb.: =A=,
breathing water; =B=, breathing air; =Si=, siphon; =T=, upper;
_t_, lower tentacles; =X=, pallial expansion, performing the part
of excurrent siphon. (After Fischer and Bouvier, x ⅓.)]
_Opisthobranchiata._--A true branchia occurs only in the
Tectibranchiata and the Ascoglossa. It lies on the right side, and is
usually more or less external, being partly covered sometimes by the
shell (as in _Umbrella_, Fig. 5), sometimes by a fold of the mantle. In
the Pteropoda (which are probably derived from the Tectibranchiata),
all the Thecosomata, with the exception of _Cavolinia_, have no
specialised branchia, but probably respire through portions or the
whole of the integument. In the Gymnosomata an accessory branchia
has in many cases been developed at the posterior end of the body.
_Pneumodermon_ alone has both lateral and posterior branchiae well
developed, _Clione_ and _Halopsyche_ are destitute of either, while the
four remaining families have one branchia, sometimes lateral, sometimes
posterior.[269]
[Illustration: FIG. 66.--_Valvata piscinalis_ Müll.: _br_,
branchia; _fi_, filament; _f.l_, foot lobes. (After Boutan.)]
[Illustration: FIG. 67.--_Doris_ (_Archidoris_) _tuberculata_ L.,
Britain: _a_, anus; _br_, branchiae, surrounding the anus; _m_,
male organ; _rh_, _rh_, rhinophores. × ⅔.]
[Illustration: FIG. 68.--_Pleurophyllidia lineata_ Otto,
Mediterranean: _a_, anus; _br_, secondary branchiae; _m_, mouth;
_s.o_, sexual orifice.]
Certain of the Nudibranchiata possess no special breathing organs, and
probably respire through the skin (_Elysia_, _Limapontia_, _Cenia_,
_Phyllirrhoë_). The majority, however, have developed secondary
branchiae, in the form of prominent lobes or leaf-like processes (the
_cerata_), which are carried upon the back, without any means of
protection. These cerata are, as a rule, of extreme beauty and variety
of form, consisting sometimes of long whip-like tentaculae, in other
cases of arborescent plumes of fern-like leafage, in others of curious
bead-like appendages of every imaginable shape and colour. In _Doris_
they lie at the posterior end of the body, in a sort of rosette, which
is generally capable of retraction into a chamber. In _Phyllidia_ and
_Pleurophyllidia_ these secondary branchiae lie, as in _Patella_, on
the lateral portions of the mantle.
The Scaphopoda in all probability possess neither true nor secondary
branchiae.
_Pulmonata._--When we use the term ‘lung,’ it must be remembered that
this organ in the Mollusca does not correspond, morphologically, with
the spongy, cellular lung of vertebrates; it simply performs the same
functions. The ‘lung,’ in the Mollusca, is a pouch or cavity, lined
with blood-vessels which are disposed over its vaulted surface in
various patterns of network. The pulmonary sac or cavity is therefore a
better name by which to denote this organ.
[Illustration: FIG. 69.--_Geomalacus maculosus_ Allm., S.
Ireland: =P.O=, pulmonary orifice.]
It seems probable, as has been already shown (pp. 18–22), that all
Pulmonata are ultimately derived from marine forms which breathed
water by means of branchiae. Thus we find intermediate forms, such as
_Siphonaria_, possessed of both a branchia and a pulmonary sac, the
former being evanescent, while in _Gadinia_ and _Amphibola_ it has
quite disappeared. In the vast majority of _Pulmonata_ no trace of
a branchia remains; its function is performed by a chamber, always
situated at the right side of the animal, and generally more or less
anterior, admitting air by a narrow aperture which is rhythmically
opened and closed. In _Arion_ and _Geomalacus_ (Fig. 69) this aperture
is in the front of the right side of the ‘shield,’ in _Limax_ (Fig. 71)
in the hinder part, in _Testacella_ (Fig. 20) it is near the extremity
of the tail, under the spire of the shell; in _Janella_ it is on
the middle of the right edge of the shield (Fig. 70). If a specimen
of _Helix aspersa_, or better, of _H. pomatia_, is held up to the
light, the beautiful arborescent vessels, with which the upper part
of the pulmonary chamber is furnished, can be clearly seen by looking
through the aperture as it dilates. It is only in the Auriculidae
that an actual spongy mass of lung material appears to exist. When
in motion, a _Helix_ inspires air much more frequently than when at
rest. Temperature, too, seems to affect the number of inspirations; it
appears doubtful whether, during hibernation, a snail breathes at all.
In any case, the amount of air required to sustain life must be small.
[Illustration: FIG. 70.--_Janella hirudo_ Fisch., N. Caledonia:
=G=, generative orifice; =P=, pulmonary orifice; =T=, =T=,
tentacles. (After Fischer.)]
[Illustration: FIG. 71.--_Limax maximus_ L.: =PO=, pulmonary
orifice. × ⅔.]
With regard to the respiration of fresh-water Pulmonata there appears
to be some difference of opinion. It is held, on the one hand, that
the Limnaeidae only respire air, making periodic visits to the surface
to procure it, and that they perish, if prevented from doing so, by
asphyxiation. If, we are told,[270] as a _Limnaea_ is floating on
the surface of the water in a glass jar, a morsel of common salt be
dropped upon its outstretched foot, it will sink heavily to the bottom,
emitting a stream of air from its pulmonary orifice. On recovering
from the shock, it will anxiously endeavour to regain the surface, but
will have some difficulty in doing so, owing to its now much greater
specific gravity. When it succeeds, it creeps almost out of the
water, and exposes its respiratory orifice freely to the air. If the
experiment is repeated several times on the same individual, it becomes
so much weakened that it has to be taken out of the water to save its
life. Moquin-Tandon, on the other hand, is strongly of opinion[271]
that there is no absolute necessity for _Limnaea_ to obtain air by
rising to the surface, and that, if prevented from emerging, it can
obtain air from the water. When covered in by a roof of ice, _Limnaea_
has not been observed to suffer any inconvenience. Moquin-Tandon kept
_L. glabra_ and _Planorbis rotundatus_ in good health under 20 mm.
of water for eighteen and nineteen days, and relates a case in which
_Physa_ was kept alive under water for four days, and _Planorbis_ for
twelve. Young specimens, both of _Limnaea_ and _Planorbis_, do not rise
to the surface for a supply of air; they are hatched with the pulmonary
cavity full of water.
It is probable, therefore, that Limnaeidae are capable, on occasion,
of respiration through the skin. Some authorities are of opinion that
certain long and narrow lamellae, situated within the pulmonary sac,
are employed for the purpose of aqueous respiration. _Ancylus_, which
never makes periodic excursions to the surface, perhaps respires by
receiving into its pulmonary chamber the minute quantities of oxygen
given off by the vegetation on which it feeds.
Limnaeidae taken from a great depth of water, _e.g._ from 130 fathoms
in the lake of Geneva, have been examined by Forel.[272] The pulmonary
sac is full of water, but there is no transformation of organs, no
appearance of a branchia, to meet the changed circumstances of their
environment. Doubtless a good deal of respiration is done by the skin;
being soft and vascular, it respires the air dissolved in the water.
Forel cites cases of _Limnaea_ living at much shallower depths, which
come to the surface once, and then remain below for months. The oxygen
of this supply must soon have become exhausted, and the animals,
discontinuing for a time the use of the pulmonary chamber, must have
respired through the skin. Shallow-water _Limnaea_, according to the
same authority, remain beneath the surface during cold weather; when
warm weather returns they rise to the surface to take in a supply of
air. Since the water at great depths is always very cold, there is no
need for the _Limnaea_ living there to rise to the surface at all.
It is a curious fact that _Limnaea_, which have been respiring by
the skin for the whole winter, should suddenly, on the first warm
days of summer, take to rising to the surface and breathing air. But
exactly the same phenomenon is shown in the case of _Limnaea_ from
great depths. Placed in an aquarium, they _immediately_ begin rising
to the surface and inspiring air; in other words, they experience
instantaneously a complete transformation of their respiratory system.
In _Onchidium_, a land pulmonate which has retrogressed to an
amphibious or quasi-marine mode of life, there is no organ which
represents the pulmonary or branchial cavity, the so-called lung being
only a cavity of the kidney. Respiration is, however, conducted by the
skin as well, and by the dorsal papillae.[273]
Land Mollusca can sustain, for a considerable time, complete
deprivation of atmospheric air. Helices placed in an exhausted receiver
show no signs of being inconvenienced for about 20 hours, and are able
to survive for about two or three days. If detained under water, they
are very active for about 6 hours, then become motionless, the body
swells, owing to the water absorbed, and death ensues in about 36
hours. Immersion for only 24 hours is generally followed by recovery.
In the latter case, the cause of death is not so much deprivation
of air as compulsory absorption of water by the skin. The amount of
water thus taken up is surprising. Spallanzani found that a _Helix_
which weighed 18 grammes increased in weight by 13½ grammes after a
prolonged immersion. Even slugs enclosed in moist paper gained more
than 2 grammes in the course of half an hour. Experiment has shown
that the amount of carbonic acid gas produced by respiration stands in
direct relation to the amount of food consumed. Four pairs of snails
were taken which had recently awakened from their winter sleep and had
eaten heartily, and an equal number, under the same circumstances,
which had been prevented from eating. It was found that the first
four pairs produced, in consuming a given amount of oxygen, 11, 9,
10, and 13 parts respectively of carbonic acid, while the second
set produced, in consuming the same amount of oxygen, only 4, 8, 7,
and 9 parts of carbonic acid.[274] Hibernating Helices, if weighed
in December and again in April, will be found to have lost weight,
due to the expiration of carbonic acid. Owing to the difficulty of
experiment, opinions vary as to the absolute temperature of snails. It
appears to be established that several snails, if placed together in
a tube, raise the temperature one or two degrees C., but as a rule,
the temperature of a solitary _Helix_ differs very slightly from that
of the surrounding air. Increased activity, whether in respiration or
feeding, is found to raise the temperature.
[Illustration: FIG. 72.--_Cardium edule_ L.: =A=, anal; =B=,
branchial siphon; =F=, foot. (After Möbius.)]
W. H. Dall, writing of the branchia in _Pelecypoda_, remarks[275]
that there can be no doubt that its original form was a simple
pinched-up lamella or fold of the skin or mantle. This, elongated,
becomes a filament. Filaments united by suitable tissue, trussed,
propped, and stayed by a chitinous skeleton, result in the forms,
wonderful in number and complexity, which puzzle the student to
describe, much more to classify.
[Illustration: FIG. 73.--_Scrobicularia piperata_ Gmel., in its
natural position in the sand: =A=, efferent or anal siphon; =B=,
afferent or branchial siphon. (After Möbius.)]
In Pelecypoda the branchiae are placed on each side of the body,
between the mantle and the visceral mass. They lie in a chamber known
as the _branchial cavity_. Leading into this cavity, and behind it,
are, as a rule, two tubes or siphons, one of which conducts water to
the branchiae, while the other carries it away after it has passed over
them. The lower is known as the _branchial_ or _afferent_ siphon, the
upper as the _anal_ or _efferent_ siphon (see Figs. 72 and 73). The
action of these siphons can readily be observed by placing a little
carmine in water, near to the siphonal apertures of an _Anodonta_ or
_Unio_. In many cases (_e.g._ _Psammobia_, _Tellina_, _Mya_, genera
which burrow deeply in sand) both the siphons are exceedingly long,
sometimes considerably longer than the whole shell. In some cases the
two tubes are free throughout their entire length, in others they
become fused together before their entrance within the shell (Fig.
74). In other genera, which do not burrow (_e.g._ _Ostrea_, _Pecten_,
_Arca_, _Mytilus_), the siphons are rudimentary or altogether absent
(Fig. 75).
[Illustration: FIG. 74.--_Solecurtus strigillatus_ L., Naples:
_s.af_, afferent siphon; _s.ef_, efferent siphon, the two uniting
in _SS_ externally to the shell, × ½.]
[Illustration: FIG. 75.--_Mytilus edulis_ L., attached by its
byssus (=By=) to a piece of wood: =F=, foot; =S=, anal siphon,
the branchial siphon being below it and not closed. (After
Möbius.)]
The number and arrangement of the branchiae varies considerably. It
appears probable that the different degrees of complication of the
gill indicate degrees of specialisation in the different groups of
Pelecypoda, in other words, assuming that a simpler form of gill
precedes, in point of development, a more complicated form, the nature
of the gill may be taken as indicating different degrees of removal
from the primitive form of bivalve.
1. The simplest form of gill (_Nucula_, _Leda_, _Solenomya_, etc.)
is that which consists (Fig. 76, A, compare Fig. 100, p. 201) of two
rows of very short, broad, not reflected filaments, the rows being
placed in such a way that they incline at right angles to one another
from a common longitudinal axis. The filaments are not connected with
one another, nor are the two leaves of each gill united at any point.
(_Protobranchiata._)
[Illustration: FIG. 76.--Morphology of the branchiae
of Pelecypoda, seen diagrammatically in section:
=A=, _Protobranchiata_; =B=, _Filibranchiata_; =C=,
_Eulamellibranchiata_; =D=, _Septibranchiata_; _e_, _e_, external
row of filaments; _i_, _i_, internal row of filaments; _e´_,
external row or plate folded back; _i´_, internal row folded
back; _f_, foot; _m_, mantle; _s_, septum; _v_, visceral mass.
(From A. Lang.)]
[Illustration: FIG. 77.--Four gill filaments of _Mytilus_, highly
magnified; _cj_, ciliary junctions; _f_, filament. (After Peck.)]
2. In the _Anomiidae_, _Arcadae_, _Trigoniidae_, and _Mytilidae_ each
gill consists of two plates or rows of much longer filaments, which
consequently occupy a much larger space in the mantle cavity (Fig.
76, B). Unable to extend beyond the limits of the mantle, filaments
are reflected or doubled back upon one another, those of the external
plate being reflected towards the outside, those of the internal plate
towards the inside. Each separate filament is not connected with the
filament next adjacent, except by surface cilia situated on small
projections on the sides of the filaments, and interlocking with the
cilia of the adjacent filament. The two superposed plates or leaves
of the gill may or may not be united by cords running between the two
parts of a filament. (_Filibranchiata._)
3. In the _Pectinidae_, _Aviculidae_, and _Ostreidae_ a further
development takes place. The filaments of each gill are reflected in
the same way as in the _Filibranchiata_, but the part thus reflected
may become completely united or ‘concresce’ with the mantle on the
exterior and with the base of the foot on the interior side. The leaves
of each gill plate, which have thus become doubled (the gills being
apparently two instead of one on each side), are folded or crumpled,
and the filaments are modified at the re-entrant angles of the fold.
(_Pseudolamellibranchiata._)
4. In all the remaining _Pelecypoda_, except class 5, in other words,
in the very large majority of families, the filaments are either
reflected, as in (3), or simple; but the process of concrescence is
so far advanced that the adjacent filaments are always intimately
connected with one another in such a way as to admit the passage of the
blood; and the leaves of each gill-plate (Fig. 76, C) are united by
cross channels in a similar way. (_Eulamellibranchiata._)
5. In certain of the _Anatinacea_ alone (_Cuspidaria_, _Lyonsiella_,
_Poromya_, _Silenia_) the gills are transformed into a more or less
muscular partition, extending from one adductor muscle to the other
(Fig. 76, D), and separating off the pallial chamber into two distinct
divisions, which communicate by means of narrow slits in the partition.
(_Septibranchiata._)
[Illustration: FIG. 78.--Transverse section of portion of an
outer gill plate of _Anodonta_, highly magnified: _il_, inner
lamella; _il´_, outer lamella; _ilj_, interlamellar junctions;
_v_, large vertical vessels. (After Peck.)]
Thus the process of gill development in the Pelecypoda appears to lead
up from a simple to a very complex type. In its original form, at all
events in the most primitive form known to us, the gill is a series of
short filaments, quite independent of one another, strung in two rows;
then the filaments become longer and double back, while at the same
time they begin to show signs of adhesion, as yet only superficial, to
one another. In a further stage, the reflected portions become fused to
the adjacent surfaces of the foot and mantle, while the interlamellar
junctions serve to lock the two gill-plates together; finally, the
mere ciliary junction of adjacent filaments is exchanged for intimate
vascular connection, while the gill-plates as a whole become closely
fused together in a similar manner.
This theory of origin is strengthened by closer observation of the
phenomena of a single group. Taking the Septibranchiata as an instance,
we find that in _Lyonsiella_ the branchiae unite with the mantle
in such a way as to form two large pallial chambers, the structure
of the branchiae being preserved, and their lamellae covering the
partition. A further stage is observed in _Poromya_. There, a similar
partition exists, but it has become muscular, preserving, however, on
each side two groups of branchial lamellae, separated one from the
other by a series of slits, which form a communication between the
two pallial chambers. A further stage still is seen in _Silenia_.
There the same muscular partition exists, but the branchial lamellae
on either side have disappeared, the slits between the two chambers,
which occur in _Poromya_, still persisting, but separated into three
groups. _Cuspidaria_ represents the last stage in the development. In
the ventral chamber there appears nothing at all corresponding to a
branchia; the surface of the partition appears perfectly uniform, but
on careful examination three little separate orifices, remains of the
three groups of orifices in _Silenia_, are observed.[276]
_Relation between Branchiae and Heart._--The object of the branchiae
being, as has been already stated, to aerate the blood on its way to
the heart, we find that the heart and the branchiae stand in very
important structural relations to one another. When the branchiae
are in pairs, we find that the auricles of the heart are also
paired, the auricle on the right and left sides being supplied by
the right and left branchiae respectively. This is the case with the
Dibranchiate Cephalopods (_Argonauta_, _Octopus_, _Loligo_, etc.),
the Zygobranchiate Prosobranchs (_Fissurella_, _Haliotis_), and all
_Pelecypoda_. In the _Amphineura_ (_Chiton_, etc.) there are two
auricles corresponding to the two sets of multiple branchiae. In the
case of the Tetrabranchiate Cephalopods (_Nautilus_) there are four
auricles corresponding to each of the four branchiae. Compare Fig. 79,
A, B, C, D, E.
On the other hand, when the branchia is single, or when both branchiae
are on the same side, and one is aborted and functionless, the auricle
is single too, and on the same side as the branchia. This is the case
with the Tectibranchiate Opisthobranchs (_Philine_, _Scaphander_,
etc.), all the Pectinibranchiate Prosobranchs (Rachiglossa,
Taenioglossa, and Ptenoglossa), and the other Azygobranchiate
Prosobranchs (Trochidae, Neritidae, etc.). In the last case the right
auricle exists, as well as the left, but is simply a closed sac, the
coalescing of the two gills on the left side having thrown all the work
upon the left auricle. Compare Fig. 79, F, G, H.
[Illustration: FIG. 79.--Diagram illustrating the relations
between branchiae, heart, and aorta in the Mollusca: =A=, In
Chiton; =B=, Pelecypoda; =C=, Dibranchiate Cephalopoda; =D=,
Tetrabranchiate Cephalopoda; =E=, Prosobranchiata Zygobranchiata;
=F=, Prosobranchiata Azygobranchiata; =G=, Prosobranchiata
Monotocardia; =H=, Opisthobranchiata Tectibranchiata: 1,
Ventricle; 2, Auricle; 3, Aorta; 3_a_, Cephalic aorta; 3_b_,
Visceral aorta; 3_c_, Posterior aorta. (From A. Lang.)]
=Circulatory System=
All Mollusca, without exception, possess a circulatory system of more
or less complexity. The centre of the system is the heart, which
receives the aerated blood from the breathing organs, and propels it to
every part of the body. In the Scaphopoda alone there appears to be no
distinct heart.
The heart may consist simply of a single auricle and ventricle, and
an aorta opening out of the ventricle. From the aorta the blood is
conveyed to the various parts of the body by arteries. Veins convey the
blood back to the breathing organs, after passing over which it returns
by the branchial or pulmonary vein to the heart, thus completing the
circuit.
As regards position, the heart is situated within the pericardium,
a separate chamber which in the Pelecypoda, Cephalopoda, and the
bilaterally symmetrical Gasteropoda lies on the median line, while in
the asymmetrical Gasteropoda it is on one or other of the sides of the
body, usually the right. The veins connected with the branchiae, and
consequently the auricle into which they open, are situated _behind_
the ventricle in the Opisthobranchiata (whence their name), while in
the Prosobranchiata they are situated _in front of_ the ventricle.
The number of auricles corresponds to the number of branchiae. Thus
there is only one auricle in the great majority of Prosobranchiata
(which are accordingly classified as _Monotocardia_), and also in
the Opisthobranchiata, while the Pulmonata have a single auricle
corresponding to the pulmonary chamber. There are two auricles in
the Amphineura, in a small group of Gasteropoda, hence known as
_Diotocardia_, in all Pelecypoda, and in the Dibranchiate Cephalopoda.
In the Tetrabranchiate Cephalopoda alone there are four auricles
corresponding to the four branchiae.
A single aorta occurs only in the Amphineura and in the Tetrabranchiate
Cephalopoda. In all the other groups there are two aortae, leading
out of the anterior and posterior ends of the ventricle in Pelecypoda
and Dibranchiate Cephalopoda, while a single aorta leads out of the
posterior end alone, and subsequently bifurcates, in most of the
Gasteropoda. One aorta, the cephalic, supplies the front part of the
body, the oesophagus, stomach, mantle, etc.; the other, the visceral
aorta, supplies the posterior part, the liver and sexual organs.
The general circulatory system in the Mollusca has not yet been
thoroughly investigated. As a general rule, the blood driven from the
ventricle through the aorta into the arteries, passes, on reaching
the alimentary canal and other adjacent organs, into a number of
irregular spaces called _lacunae_. These in their turn branch into
_sinuses_, or narrow tubes covered with muscular tissue, which
penetrate the body in every direction. In the Dibranchiate Cephalopoda
true capillaries are said to occur, which in some cases form a direct
communication between the arteries and veins. According to some
authorities[277] capillaries and veins exist in certain Pelecypoda in
connexion with the intestinal lacunae, but this again is regarded by
others as not established. A similar difference of opinion occurs with
regard to the precise function of the foot-pore which occurs in many
Mollusca, some holding that it serves as a means for the introduction
of water into the blood-vascular system, while others regard it as a
form of secretion gland, the original purpose of which has perhaps
become lost.
=Blood.=--As a rule, the blood of the Mollusca--_i.e._ not the
corpuscles but the liquor sanguinis--is colourless, or slightly tinged
with blue on exposure to the air. This is due to the presence of a
pigment termed _haemocyanin_, in which are found traces of copper and
iron, the former predominating. _Haemoglobin_, the colouring matter
of the blood in Vertebrates, is, according to Lankester,[278] of very
restricted occurrence. It is found--(1) in special corpuscles in the
blood of _Solen legumen_ (and _Arca Noae_); (2) in the general blood
system of _Planorbis_; (3) in the muscles of the pharynx and jaws
of certain Gasteropoda, _e.g._ _Limnaea_, _Paludina_, _Littorina_,
_Chiton_, _Aplysia_. This distribution of haemoglobin is explained by
Lankester in reference to its chemical activity; whenever increased
facilities for oxidisation are required, then it may be present to
do the work. The Mollusca, being as a rule otiose, do not possess it
generally diffused in the blood, as do the Vertebrata. The actively
burrowing _Solen_ possesses it, and perhaps its presence in _Planorbis_
is to be explained from its respiring the air of stagnant marshes.
Its occurrence in the pharyngeal muscles and jaws of other genera may
be due to the constant state of activity in which these organs are
kept.[279]
According to Tenison-Woods[280] a species of _Arca_ (_trapezia_ Desh.)
and two species of _Solen_, all Australian, have red blood. It is
suggested that in these cases the habits of the animal (the _Solen_
burrowing deeply in sand, the _Arca_ in mud) require some highly
oxidising element, surrounded as the creature is by ooze. In _Arca
pexata_ (N. America) the blood is red, the animal being familiarly
known as the ‘bloody clam.’ Burrowing species, however, are not all
distinguished by this peculiarity. Tenison-Woods finds red fluids in
the buccal mass of many Gasteropoda, _e.g._ in species of _Patella_,
_Acmaea_, _Littorina_, _Trochus_, _Turbo_, giving the parts the
appearance of raw meat.
=The Mantle=
On the dorsal side of the typical molluscan body, between the visceral
sac and the shell, lies a duplicature of the integument, generally
known as the _mantle_. The depending sides of the mantle, which are
usually somewhat thickened, enclose between themselves and the body
mass a chamber of varying size and shape, called the _mantle cavity_,
which communicates freely with the external air or water, and encloses
and furnishes a protection for the organ or organs of respiration. On
its upper or dorsal surface the mantle is closely applied to the shell
throughout its whole extent, the cells with which it is furnished
secreting the materials from which the shell is formed (see p. 255).
The whole mantle is capable, to some degree, of secreting shelly
matter, but the most active agent in its production is the mantle edge
or margin.
In the Prosobranchiata the mantle cavity, for reasons which have
already been explained, is found on the left side of the animal, its
front portion being in many cases produced into a tubular siphon.
Within the mantle cavity are found, besides the branchia, the anus,
the apertures of the kidneys, and the osphradium. In the pulmonata the
mantle fold encloses a so-called lung-cavity. The front edge of the
mantle coalesces with the integument of the neck in such a way as to
enclose the cavity very completely, the only communication with the
outer air being by means of the contractile breathing or pulmonary
aperture on the right side. In the Tectibranchiate Opisthobranchs the
mantle fold is inconsiderable, and is usually not of sufficient extent
to cover the branchia, while in the Nudibranchs, which have no true
branchiae, it disappears altogether.
In the Pelecypoda the mantle cavity is equally developed on each side,
enclosing the two sets of branchiae. The mantle may thus be regarded as
consisting of two equal portions, which form a sort of lining to the
two valves. The lower or ventral portion of the mantle edges may be
simple, or provided with ocelli (_Pecten_, _Arca_), tentacles, cilia
(_Lima_, _Lepton_), or doubled folds. The two portions of the mantle
touch one another along the whole line of the edge of the two valves,
and, although thus in contact, may remain completely separate from one
another, or else become permanently united at one or more points. This
fusion of the mantle edges corresponds to important changes in the
organisation of the animal as a whole. The anal and branchial siphons
are no more than prolongations of the mantle edges on the posterior
side into a tubular form. These ‘siphons’ exhibit the siphonal form
more distinctly according as the adjacent portions of the mantle become
more definitely fused together.
[Illustration: FIG. 80.--Diagram illustrating the various
stages in the closing of the mantle in Pelecypoda: =A=, mantle
completely open; =B=, rudiments of siphons, mantle still
completely open; =C=, mantle closed at one point; =D=, mantle
closed at two points, with complete formation of siphonal
apertures; =E=, development of siphons, ventral closure more
extended; =F=, mantle closed at three points, with fourth
orifice: _f_, foot; _s.a_, _s.b_, anal and branchial siphons;
1, 2, 3, first, second, and third points of closure of mantle.
(After A. Lang.)]
This progressive fusion of the mantle edges may be taken as indicating
definite stages in the development of the Pelecypoda. A perfectly free
mantle edge, joined at no point with the edge of the adjacent mantle,
occurs in _Nucula_, _Arca_, _Anomia_, and _Trigonia_ (see Fig. 80,
A, B). Here there is nothing in the nature of a siphon, either anal
or branchial; in other words, no contrivance exists to prevent the
spent water which has passed over the branchiae from becoming mixed
with the fresh water which is to reach them. When the mantle edges
are fused at _one point only_, this is invariably on the middle part
of the posterior side, thus separating off an anal opening which may
become prolonged into a tube-like form. At the same time the adjacent
underlying portions of the mantle edges draw together, without actually
coalescing, to form an opening for the incurrent stream of water,
the rudiments of the ‘branchial siphon’ (Fig. 80, C). This is the
case with most _Mytilidae_ (see Fig. 75) with _Cardita_, _Astarte_,
and _Pisidium_. In the next stage the branchial opening is separated
off by the concrescence of the mantle edges beneath it, and we have
the mantle united _in two places_, thus forming three openings, the
ventral of which is the opening for the protrusion of the foot (Fig.
80, D). This is the case in _Yoldia_, _Leda_, the majority of the
Eulamellibranchiata (_e.g._ _Lucina_, _Cyrena_, _Donax_, _Psammobia_,
_Tellina_, _Venus_, _Cardium_, _Mactra_), and all Septibranchiata.
In _Chama_ and _Tridacna_ the fused portions of the mantle become
more extended, and in _Pholas_, _Xylophaga_, _Teredo_, _Pandora_, and
_Lyonsia_ this concrescence takes place over the greater length of the
whole mantle edge, so that the mantle may be regarded as closed, with
the exception of the three apertures for the foot and the two siphons
(Fig. 80, E).
In certain genera there occurs, besides these three apertures, a
fourth, in the line of junction between the pedal and branchial
orifices. It appears probable that this fourth orifice (which has been
regarded by some as an inlet for water when the siphons are retracted),
stands in relation to the byssal apparatus (Fig. 80, F). In _Lyonsia_,
for instance, a thick byssus protrudes through the orifice, which is
large and open. In _Solen_, _Lutraria_, _Glycimeris_, _Cochlodesma_,
_Thracia_, _Aspergillum_, and a few more genera, which have no byssus,
the orifice is very small and narrow. It is possible that in these
latter cases, the byssal apparatus having become atrophied, the orifice
has been correspondingly reduced in size.[281]
=Mantle Reflected over the Shell.=--It is sometimes the case that the
mantle edges tend to double back over the external surface of the
shell, and to enclose it to a greater or less extent. When this process
is carried to an extreme, the edges of the reflected mantle unite, and
the shell becomes completely internal. We see an incipient stage of
this process in _Cypraea_ and _Marginella_, where the bright polish
on the surface of the shell is due to the protection afforded by the
lobes of the mantle. A considerable portion of the shell of _Scutus_
is concealed in a similar way, while in _Cryptochiton_, _Lamellaria_,
and _Aplysia_ the shell is more or less completely enclosed. Among
Pulmonata, it is possible that in forms like _Vitrina_, _Parmacella_,
_Limax_, and _Arion_, we have successive stages in a process which
starts with a shell completely external, as in _Helix_, and ends, not
merely by enveloping the shell in the mantle, but by effecting its
disappearance altogether. In _Vitrina_ and some allied genera we
have a type in which the mantle lobes are partly reflected over the
shell, which at the same time exhibits rather less of a spiral form
than in _Helix_. In the stage represented by _Parmacella_, the mantle
edges have coalesced over the whole of the shell, except for a small
aperture immediately over the spire; the nucleus alone of the shell
is spiral, the rest considerably flattened. In _Limax_ the shell has
become completely internal, and is simply a flat and very thin plate,
the spiral form being entirely lost, and the nucleus represented by a
simple thickening at one end of the plate. In _Arion_, the final stage,
we find that the shell, being no longer needed as a protection to the
vital organs, has either become resolved into a number of independent
granules, or else has entirely disappeared.
Some indications of a similar series of changes occur in the
Pelecypoda. The mantle edge of _Lepton_ is prolonged beyond the area
of the valves, terminating in some cases in a number of filaments. In
_Galeomma_ and _Scintilla_ the valves are partially concealed by the
reflected mantle lobes, and in a remarkable form recently discovered
by Dall[282] (_Chlamydoconcha_) the shell is completely imbedded in
the mantle, which is perforated at the anterior end by an orifice for
the mouth, and at the posterior end by a similar orifice for the anus.
In all these cases, except _Lepton_, it is interesting to notice that
the hinge teeth have completely disappeared, the additional closing
power gained by the external mantle rendering the work done by a hinge
unnecessary. It is quite possible, on the analogy of the Gasteropoda
mentioned above, and also, it may be added, of the Cephalopoda and
other groups, that we have here indicated the eventual occurrence of a
type of Pelecypoda altogether deprived of valves, a greatly thickened
mantle performing the part of a shell.[283]
* * * * *
The following works will be found useful for further study of this
portion of the subject:--
=F. Bernard=, Recherches sur les organes palléaux des
Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7) ix. (1890),
pp. 89–404.
=G. Cuvier=, Le Régne animal (ed. V. Masson); Mollusca, Text and
Atlas.
=C. Grobben=, Beiträge zur Kenntniss des Baues von Cuspidaria
(Neaera) cuspidata Olivi, nebst Betrachtungen über das System
der Lamellibranchiaten: Arb. Zool. Inst. Wien, x. (1893), pp.
101–146.
=E. Ray Lankester=, Encyclopaedia Britannica, 9th ed., vol. xvi.
(1883), Art. ‘Mollusca.’
=A. Ménégaux=, Recherches sur la circulation des Lamellibranches
marins: Besançon, 1890.
=K. Mitsukuri=, On the structure and significance of some
aberrant forms of Lamellibranchiate gills: Q. Journ. Micr. Sc.,
N.S. xxi. (1881), pp. 595–608.
=H. L. Osborn=, On the gill in some forms of Prosobranchiate
Mollusca: Stud. Biol. Lab. Johns Hopk. Univ. iii. (1884), pp.
37–48.
=R. Holman Peck=, The structure of the Lamellibranchiate gill:
Q. Journ. Micr. Sc., N.S. xvii. (1877), pp. 43–66.
=P. Pelseneer=, Contributions à l’étude des lamellibranches:
Arch. Biol. xi. (1891), pp. 147–312.
CHAPTER VII
ORGANS OF SENSE: TOUCH, SIGHT, SMELL, HEARING--THE
FOOT--THE NERVOUS SYSTEM
_Organs of Sense_: I. _Touch_
Tactile organs, although occurring in some of the Mollusca, do not
appear to attain special or marked development, except in a few cases.
The whole surface of the skin, and particularly of the foot, is very
sensitive to the slightest impression. Nearly all Gasteropoda are
furnished with at least two cephalic tentacles, projecting like horns
from each side of the fore part of the head. At or near the base of
these are generally situated the eyes. In the Helicidae the eyes are
situated, not at the base, but at the apex of the tentacles, and in
that case--except in _Vertigo_--a second pair of shorter tentacles
appears beneath the longer pair. It frequently happens that several
senses are centred in a single organ; thus the upper tentacles of
snails not only carry the eyes and serve to a certain extent as tactile
organs, but they also carry the organs of smell.
The edges of the mantle, which are sometimes specialised into lobes,
appear to be keenly sensitive to touch in all Gasteropoda.
In _Cypraea_ (Fig. 81) these lobes, or tentaculae, are a prominent
feature of the animal, and also in certain genera of the Trochidae
(Fig. 82). In most of the carnivorous land Pulmonata--_e.g._
_Testacella_, _Rhytida_, _Ennea_--there are developed, under the lower
pair of tentacles, and close to the mouth, large labial palps or
feelers. These are connected with the cerebral ganglion by a very large
nerve, and may therefore be supposed to be of extreme sensitiveness.
In some of the large carnivorous forms (_Glandina_, _Aerope_, compare
Fig. 21, p. 54) these palpae are of great size, and curl upwards like
an enormous pair of moustaches. When a _Glandina_ seizes its prey, the
palpae (see Fig. 83) appear to enfold it and draw it in towards the
mouth.
[Illustration: FIG. 81.--_Cypraea moneta_ L., showing tentaculae
at edge of mantle, which partly envelops the shell: =Si=, siphon;
=M=, =M=, mantle; =F=, foot; =T´=, tentaculae at edge of mantle.
(After Quoy and Gaimard.) × 3/2.
[Illustration: FIG. 82.--_Monodonta canalifera_ Lam., New
Ireland, showing mantle lobes. (After Quoy and Gaimard.)]
[Illustration: FIG. 83.--_Glandina_ seizing its prey, with buccal
papillae turned back. (Strebel.)]
It is in the Opisthobranchiata that the organs of touch attain their
maximum development. Many of this group are shell-less or possess a
small internal shell, and accordingly, in the absence of this special
form of defence, a multiplied sense of touch is probably of great
service. Thus we find, besides the ordinary cephalic tentacles,
clusters or crowns of the same above the head of many Nudibranchiata,
with lobe-like prolongations of the integument, and tentacular
processes in the neighbourhood of, or surrounding the branchiae (see
Figs. 58 and 84), or even projecting from the whole upper surface of
the body (Fig. 5, C).
In the Pelecypoda, the chief organs of touch are the foot, which is
always remarkably sensitive, especially towards its point, the labial
palps on each side of the mouth, and the siphons. In certain cases the
mantle border is prolonged into a series of threads or filaments. These
are particularly noticeable in _Pecten_, _Lepton_, and _Lima_ (Fig.
85), the mantle lobes of the common _L. hians_ of our own coasts being
very numerous, and of a bright orange colour. In many genera--_e.g._
_Unio_, _Mactra_--this sensibility to touch appears to be shared
by the whole mantle border, although it is not furnished with any
special fringing. The ‘arms’ of the Cephalopoda appear to be keenly
sensitive to touch, and this is particularly the case with the front
or tentacular pair of arms, which seem to be employed in an especial
degree for exploration and investigation of strange objects.
[Illustration: FIG. 84.--_Idalia Leachii_ A. and H., British
seas; _br_, branchiae. (After Alder and Hancock.)]
[Illustration: FIG. 85.--_Lima squamosa_ Lam., Naples, showing
tentacular lobes of mantle (_t_, _t_); _a_, anus; _ad.m_,
adductor muscle; _br_, _br_, branchiae; _f_, foot; _sh_, shell.]
=Taste.=--The sense of taste is no doubt present, to a greater or less
extent, in all the head-bearing Mollusca. In many of these a special
nerve or nerves has been discovered in the pharynx, connecting with the
cerebral ganglion; this no doubt indicates the seat of the faculty of
taste. The Mollusca vary greatly in their likings for different kinds
of food. Some seem to prefer decaying and highly odoriferous animal
matter (_Buccinum_, _Nassa_), others apparently confine themselves to
fresh meat (_Purpura_, _Natica_, _Testacella_), others again, although
naturally vegetarian, will not refuse flesh on occasion (_Limax_,
_Helix_).
Mr. W. A. Gain[284] has made some interesting experiments on the taste
of British land Mollusca, as evidenced by the acceptance or rejection
of various kinds of food. He kept twelve species of _Arion_ and
_Limax_, and eight species of _Helix_ in captivity for many months, and
tried them with no less than 197 different kinds of food, cannibalism
included. Some curious points came out in his table of results.
_Amalia gagates_ appears to be surprisingly omnivorous, for out of 197
kinds of food it ate all but 25; _Arion ater_ came next, eating all but
40. _Limax arborum_, on the other hand, was dainty to a fault, eating
only seven kinds of food, and actually refusing Swedes, which every
other species took with some avidity. Certain food was rejected by all
alike, _e.g._ London Pride, Dog Rose, Beech and Chestnut leaves, Spruce
Fir, Common Rush, Liverwort, and Lichens; while all, or nearly all, ate
greedily of Potatoes, Turnips, Swedes, Lettuces, Leeks, Strawberries,
_Boletus edulis_, and common grasses. Few of our common weeds or
hedgerow flowers were altogether rejected. _Arion_ and _Limax_ were
decidedly less particular in their food than _Helix_, nearly all of
them eating earthworms and puff-balls, which no _Helix_ would touch.
_Arion ater_ and _Limax maximus_ ate the slime off one another, and
portions of skin. _Cyclostoma elegans_ and _Hyalinia nitida_ preferred
moist dead leaves to anything else.
II. _Sight_
=Position of Eyes.=--In the majority of the head-bearing Mollusca
the eyes are two in number, and are placed on, or in the immediate
neighbourhood of the head. Sometimes they are carried on projecting
tentacles or ‘ommatophores,’ which are either simple (as in
Prosobranchiata) or capable of retraction like the fingers of a
glove (_Helix_, etc.). Sometimes, as in a large number of the marine
Gasteropoda, the eyes are at the outer base of the cephalic tentacles,
or are mounted on the tentacles themselves, but never at the tip
(compare Fig. 60, p. 153 and Fig. 98, p. 199). In other cases they are
placed somewhat farther back, at the sides of the neck. The Pulmonata
are usually subdivided into two great groups, _Stylommatophora_ and
_Basommatophora_ (Fig. 86), according as the eyes are carried on the
tip of the large tentacles (_Helix_, and all non-operculate land
shells), or placed at the inner side of their base (_Limnaea_, _Physa_,
etc.). In land and fresh-water operculates, the eyes are situated at
the outer base of the tentacles.
[Illustration: FIG. 86.--=A=, _Limnaea peregra_ Müll.; _e_, _e_,
eyes; _t_, _t_, tentacles; =B=, _Helix nemoralis_ Müll.; _e_,
_e_, eyes; _t_, _t_, tentacles; _p.o_, pulmonary orifice.]
In the Helicidae, careful observation will show that the eyes are not
placed exactly in the centre of the end of the tentacle, but on its
upper side, inclining slightly outwards. The eye is probably pushed on
one side, as it were, by the development of the neighbouring olfactory
bulb. The sense of smell being far more important to these animals than
the sense of sight, the former sense develops at the expense of the
latter.
=Organisation of the Molluscan Eye.=--The eye in Mollusca exhibits
almost every imaginable form, from the extremely simple to the
elaborately complex. It may be, as in certain bivalves, no more than
a pigmented spot on the mantle, or it may consist, as in some of the
Cephalopoda, of a cornea, a sclerotic, a choroid, an iris, a lens, an
aqueous and vitreous humour, a retina, and an optic nerve, or of some
of these parts only.
In most land and fresh-water Mollusca the eye may be regarded, roughly
speaking, as a ball connected by an exceedingly fine thread (the optic
nerve) with a nerve centre (the cerebral ganglion). In _Paludina_
this ball is elliptic, in _Planorbis_ and _Neritina_ it is drawn out
at the back into a conical or pear shape. In _Helix_ (Fig. 87) there
is a structureless membrane, surrounding the whole eye, a lens, and a
retina, the latter consisting of a nervous layer, a cellular layer, and
a layer of rods containing pigment, this innermost layer (that nearest
the lens) being of the thickness of half the whole retina.
[Illustration: FIG. 87.--Eye of _Helix pomatia_ L., retracted
within the tentacle; _c_, cornea; _ep_, epithelial layer; _l_,
lens; _op.n_, optic nerve; _r_, retina. (After Simroth.)]
Comparing the eyes of different Gasteropoda together, we find that
they represent stages in a general course of development. Thus in
_Patella_ the eye is scarcely more than an invagination or depression
in the integument, which is lined with pigmented and retinal cells.
The next upward stage occurs in _Trochus_, where the depression
becomes deeper and bladder-shaped, and is filled with a gelatinous or
crystalline mass, but still is open at the top, and therefore permits
the eye to be bathed in water. Then, as in _Turbo_, the bladder becomes
closed by a thin epithelial layer, which finally, as in some _Murex_,
becomes much thicker, while the ‘eyeball’ encloses a lens (Fig. 88),
which probably corresponds with the ‘vitreous humour’ of other types.
[Illustration: FIG. 88.--Eyes of Gasteropoda, showing arrest of
development at successive stages: =A=, _Patella_; =B=, _Trochus_;
=C=, _Turbo_; =D=, _Murex_; _ep_, epidermis; _l_, lens; _op.n_,
optic nerve; _r_, retina; _v.h_, vitreous humour. (After Hilger.)]
In _Nautilus_ the eye is of a very simple type. It consists of a
cup-shaped depression, with a small opening which is not quite closed
by the integument. The retina consists of cells which line the
interior of the depression, and which communicate directly with the
branches of the optic nerve, there being no iris or lens. This type of
eye, it will be observed, corresponds exactly with that which occurs in
_Patella_. It appears also to correspond to a stage in the development
of eyes in the Dibranchiata (_e.g._ _Octopus_, _Sepia_, _Loligo_).
Lankester has shown[285] that in _Loligo_ the eye first appears as a
ridge, enclosing an oval area in the integument. By degrees the walls
of this area close in, and eventually join, enclosing the retinal cells
within the chamber in which the lens is afterwards developed (Fig.
89). It thus appears that in some cases the development of the eye is
arrested at a point which in other cases only forms a temporary stage
towards a higher type of organisation.
[Illustration: FIG. 89.--Three stages in the development of the
eye of _Loligo_; _r_, _r_, ridge, enclosing _p.o.c_, primitive
optic chamber; _or_, orifice between the closing ridges;
_s.o.c_, secondary optic chamber; _ci_, _ci_, ciliary body; _l_,
rudimentary lens; _R_, retina. (After Lankester.)]
[Illustration: FIG. 90.--Eye in =A=, _Loligo_; =B=, _Helix_ or
_Limax_; =C=, _Nautilus_: _a.o.c_, anterior optic chamber; _c_,
cornea; _int_, integument; _ir_, iris; _l_, lens; _l´_, external
portion of lens; _op.n_, optic nerve; _op.g_, optic ganglion;
_p.o.c_, posterior optic chamber; _r_, retina. (After Grenacher.)]
The developed eye in the dibranchiate Cephalopods consists of a
transparent cornea, which may or may not be closed over the front of
the lens. Behind the cornea is a narrow chamber (the anterior optic
chamber) which is continued for three parts round the whole circle
of the eye, and into which project the front portion of the lens and
the folds of the iris. Throughout its whole extent, the anterior optic
chamber is lined by the integument, the portion of which on the inner
side is the choroid. The lens is divided into an outer and inner
segment by a thin membrane, and is supported by the ciliary body which
forms a continuation of the retina. The main portion of the lens lies
within the posterior optic chamber, at the back and sides of which is
found the retina (Grenacher).
There can be no doubt that the Cephalopoda use their eyes to observe,
but there is nothing to show that any other Mollusca use their eyes
for this purpose, the sense of smell in their case largely taking the
place of visual observation. Madame Jeannette Power once saw[286]
the _Octopus_ in her aquarium holding a fragment of rock in one of
its arms, and watching a _Pinna_ which was opening its valves. As
soon as they were perfectly open, the Poulpe, with incredible address
and promptitude, placed the stone between the valves, preventing
the _Pinna_ from closing again, upon which it set about devouring
its victim. The next day the Poulpe was seen, after crushing some
_Tellina_, to stretch himself down close by a _Triton nodiferus_, and
watch it attentively. After four hours the _Triton_ emerged from its
shell, when the _Octopus_ sprang upon it, and surrounded it with its
arms.
=Powers of Vision in Land Mollusca.=--The Helicidae are undoubtedly
very short-sighted. Seldom emerging from their retreats except in
twilight and darkness, they are naturally myopic, and see better in a
subdued than in a bright light. Experiment has shown that a _Helix_ can
perceive an object better at 6 centimetres distance in a weak light
than at 4 or 5 millimetres in a strong one. _Cyclostoma elegans_ and
_Paludina vivipara_ are comparatively long-sighted, perceiving objects
at a distance of 20 to 30 centimetres.[287] The increased power of
vision is due, in these two cases, to increased elaboration in the
construction of the eye, _Paludina_ possessing a large and almost
spherical lens, to which the vitreous humour closely adheres, while in
_Cyclostoma_ the lens is remarkably hard, and the aqueous humour very
abundant. According to V. Willem,[288] the Pulmonata are very sensitive
to the slightest movement of the air or jarring of the surface on which
they crawl, but are so short-sighted as only to perceive a confused
image of a large object at about 1 cm., and to distinguish the form of
objects at not more than 1 or 2 mm. The senses of touch and smell are
far more active than that of sight. A bean-pod enclosed in a narrow
glass case and placed before a hungry snail was not noticed, but when
taken out of the case and placed 8 cm. behind the snail, the latter at
once turned towards it to devour it.
Some interesting experiments were conducted by the same author with the
view of ascertaining whether snails avoid or court the light. He placed
a number of species in different wooden boxes, which were divided into
a light and a dark compartment, having previously well soaked the boxes
in water to secure a humid atmosphere and surface, and so induce the
snails to move about. The result showed that nearly all species have a
marked predilection one way or the other, but not all in the same way.
_Helix aspersa_, _Arion empiricorum_, six species of _Limax_, and three
of _Planorbis_, are lovers of darkness, while _H. nemoralis_, _Succinea
putris_, and two species of _Limnaea_ are lovers of light. _Physa
fontinalis_ stands alone in being quite indifferent.
M. Willem endeavoured further to discover whether any of the Mollusca
possessed ‘dermatoptic perception,’ or the faculty of perceiving
variation of light by means of the skin alone. He accordingly repeated
the above-mentioned experiments, having previously extirpated the
eyes in all cases. The result was remarkable. In a few instances the
experiment was not conclusive, but _H. aspersa_, _A. empiricorum_,
several species of _Limax_, and one _Limnaea_ shunned or sought the
light just as they had done when their eyes were present. A few marine
Mollusca (_Littorina littorea_, _Trochus cinerarius_, _T. umbilicatus_,
_Patella vulgata_) were also shown to be exceedingly sensitive to the
impact of a shadow, whether with or without their eyes.
=Blind and Eyeless Mollusca.=--In a large number of marine Mollusca
which habitually creep about half buried in wet sand (_Bullia_,
_Sigaretus_, _Scaphander_, _Philine_), eyes are altogether absent. In
some species of _Natica_ and _Sigaretus_, and in _Doris_, eyes are
developed, but are enclosed in a thick layer of skin, through which
they can probably do little more than faintly appreciate different
degrees of light and darkness. _Chiton_ has cephalic eyes in the
embryo, but loses them in the adult stage. The two great _Auricula_,
_A. auris Judae_ and _A. auris Midae_, which habitually creep about
in the liquid mud of mangrove swamps, have entirely lost their eyes.
Certain pelagic Mollusca seem to have a tendency, which is not easily
explained, to lose their eyes or the power of seeing with them. Thus
_Ianthina_ has no eyes at all. Pteropoda as a rule have no eyes,
and the few that have (_Creseis_, _Cavolinia_) possess only certain
pigmented spots placed near to the nervous centres. In the Heteropoda,
however, and the Cephalopoda, many of which are pelagic, the eyes are
unusually large.
[Illustration: FIG. 91.--_Sigaretus laevigatus_ Lam., a species
frequenting wet sand, and destitute of external eyes: =F=,
anterior portion of foot. (After Souleyet.)]
=Eyes in Deep-sea and Underground Mollusca.=--Deep-sea Mollusca, as a
rule, possess no visual organs, or possess them only in a rudimentary
state, but this rule has its exceptions. Dr. Pelseneer found[289] no
trace of eyes in two species of _Pleurotoma_ from 1850 and 1950 fath.,
none in a _Fossarus_ from 1400 f., none in a _Puncturella_ from 1340 f.
A remarkable form of _Voluta_ (_Guivillea_) from 1600 f. possessed eyes
which could hardly be functional, as they were destitute of pigment,
and exhibited other changes of structure. On the other hand, it is
remarkable to notice that in three different species of _Trochus_ from
450 f., 565 f., and 1375 f., the eyes were pigmented and well developed.
In land Mollusca which live beneath the surface of the ground or in
absolute darkness, the eyes are generally more or less modified.
Thus in _Testacella_, which usually burrows deeply in the soil, but
occasionally emerges into the open air, the eyes are very small, but
distinct and pigmented. Our little _Caecilianella acicula_, which
is never seen above the surface, is altogether destitute of eyes. A
species of _Zospeum_, a _Helix_, and a _Bithynella_ from dark caves
in Carniola have suffered a similar loss. On the other hand, a small
_Hyalinia_ from a dark cave in Utah (probably a recent addition to the
cave fauna) has the eyes normally developed.
=Eyes of Onchidium.=--Many species of _Onchidium_, a naked land
pulmonate which creeps on rocks near high-water mark, are provided
with dorsal eyes of various degrees of organisation, and in numbers
varying up to nearly one hundred. The tropical _Onchidium_ are the
prey of a fish (_Periophthalmus_) which skips along the beach by the
aid of its large ventral fins, and feeds principally on insects and
_Onchidium_. Karl Semper suggests[290] that the eyes are of service to
_Onchidium_ as enabling it to apprehend the shadow of the approaching
_Periophthalmus_, and defend itself by suddenly contracting certain
glands on the skin and expressing a liquid secretion which flies
into the air like shot and frightens the _Periophthalmus_ away. This
theory for it is no more than theory--may or may not be true, but it
is remarkable that _Onchidium_ with dorsal eyes have precisely the
same geographical distribution as _Periophthalmus_, and that where no
_Periophthalmus_ exists, _e.g._ on our own S.W. coasts, the _Onchidium_
are entirely destitute of dorsal eyes. In those species of _Onchidium_
which have no dorsal eyes, the latter are on the tips of the tentacles,
as in _Helix_. The eyes are developed _on_ the head, and afterwards
ascend with the growth of the ommatophores, while in _Helix_ the
ommatophores are formed first, and the eyes developed upon them.[291]
=Dorsal Eyes in the Chitonidae.=--The remarkable discoveries of Moseley
with regard to the dorsal eyes of _Chiton_ were first published in
1884.[292] He happened to notice, while examining a specimen of
_Schizochiton incisus_, a number of minute black dots on the outer
surface of the shell, which appeared to refract light as if composed
of glass or crystal. These ‘eyes,’ in all the species of _Chiton_ yet
examined, are restricted to the outer surface of the exposed area of
the shell, never being on the laminae of insertion or on the girdle.
In certain sub-genera of _Chiton_ the eyes are scattered irregularly
over the surface, in others they are arranged symmetrically in rows
diverging from the apex of each plate, but in old specimens the eyes
towards the apices are generally rubbed off by erosion or abrasion.
Moseley regarded the occurrence of scattered eyes as indicating an
original stage of development, when the eyes were at first disposed
irregularly all over the surface of the shell; the gathering into
regular rows showing a later stage.
[Illustration: FIG. 92.--Dorsal eyes of _Chitonidae_, showing the
various forms of arrangement in the first and fourth valves of 1,
1_a_, _Acanthopleura spinigera_ Sowb., E. Indies, × 2; 2, 2_a_,
_Tonicia suezensis_ Reeve, Suez, × 3; 3, 3_a_, _Acanthopleura
granulata_ Gmel., W. Indies, × 2; 4, 4_a_, _Tonicia lineolata_
Fremb., Chili, × 2. From specimens in the Museum of Zoology,
Cambridge.]
The eyes appear to be invariably more numerous on the anterior plate.
Thus in _Corephium aculeatum_ there are about 12,000 in all, of which
more than 3000 are on the anterior plate. In _Schizochiton_ they are
arranged in very symmetrical rows, six of which are situated on the
anterior, and only two, sometimes only one, on the central plates. In
_Tonicia marmorata_ the eyes are sunk in little cup-shaped depressions
of the shell, possibly to escape abrasion. As regards shape and size,
in _Ch. incisus_ they are circular, and about 1/35 inch in diameter,
this being the largest size known; in _Ch. spiniger_ and _Ch.
aculeatus_ they are oval, measuring about 1/400 x 1/600 inch. There are
no eyes in _Chiton_ proper, nor in _Mopalia_, _Maugeria_, _Lorica_,
and _Ischnochiton_.[293] None of our English species appear to possess
them.[294]
=Eyes in Bivalve Mollusca.=--Some, possibly most, of the Pelecypoda
possess, in the larval state, true paired eyes at the oral end of the
body. These become aborted as the animal develops, since that part of
the body becomes entirely screened from the light by the growth of the
shell. To compensate for their loss, numerous _ocelli_, or pigmented
spots sensitive to the action of light, are in many cases developed
on different parts of the mantle, functionally corresponding to the
‘eyes’ of _Chiton_ described above. As in _Chiton_, too, we have here
an interesting series of instances in which true eyes have suffered
total obliteration, through disuse, and, as if to restore to the animal
in some measure its lost sense, visual organs of a low power have
subsequently been developed and are now observed in various stages of
specialisation.
=Concentration of Eyes in Special Parts of the Mantle.=--Sharp has
shown[295] that in several species of _Ostrea_, _Cardium_, _Anomia_,
_Lima_, _Avicula_, _Arca_, and _Tellina_ pigmented cells, with a
highly refractive cuticle, are scattered over a considerable portion
of the mantle. Experiment has proved the powers of ‘vision,’ _i.e._
of sensitiveness to different degrees of light, possessed by these
organs. In _Dreissena polymorpha_, _Tapes decussatus_, and two species
of _Venus_ these cells are concentrated on that particular part of the
mantle which is not always covered by the shell, _i.e._ the siphon,
but since the siphon can be completely retracted within the shell,
there is no special provision for their protection. A further step
is shown in the case of _Mya arenaria_, where the siphon is scarcely
capable of complete retraction. Here, while some of the pigment cells
are scattered about over the surface of the siphon, the majority
are placed in grooves at the base of the siphonal tentacles, forming
an intensely black band round them. A higher stage still is shown in
_Solen vagina_, _S. ensis_, and _Mactra solidissima_, where the cells
are situated only in the siphonal grooves, which are more or less
specialised in numbers and complexity.
_Arca Noae_, according to Patten, is very sensitive to any sudden
change in the amount of light falling upon its mantle-edge. A faint
shadow cast upon it by the hand is sufficient to cause it to close
its valves quickly, but always one or two seconds afterwards, the
promptitude in all cases depending upon the depth of the shadow.
Sensitiveness in this direction was found to depend greatly upon
the vitality of the animals themselves, since it always became less
in those specimens which had been kept for long in confinement. A
shadow was not always necessary to make them close. An ordinary
black pencil, if approached within two or three inches with extreme
caution, produced the same result, while a glass rod brought within
the same distance, and even moved rapidly to and fro, appeared to
cause no alarm. Sensitiveness to change in intensity of light was
experimentally noticed by the same author in the case of _Ostrea_,
_Mactra_, _Avicula_ (to a special extent), and _Cardium_. It is very
remarkable to find that increased elaboration in the structure of the
eyes does not necessarily carry with it increased sensitiveness, _i.e._
higher visual powers. _Avicula_, which is only provided with a few
scattered ommatidia, which would entirely escape the notice of any one
who had not seen them better developed elsewhere, was considerably more
sensitive to light and shade than _Arca_, with its eyes of conspicuous
size and much more perfect organisation, instantly contracting the
mantle upon the impact of a shadow so faint as to be invisible to the
experimenter.[296]
=Visual Faculties of Solen and Ostrea.=--The visual power of _Solen_
may be exemplified by any one who is walking along almost any of our
sandy bays at extreme low-water mark. If the day be warm and sunny,
numbers of _Solen_ will be seen raising themselves an inch or two out
of their holes; but if you wish to catch them you must approach very
cautiously, and on no account allow your shadow to fall upon them, or
they will pop down into their burrows in an instant, and it is vain
to attempt to dig them out. ‘How sensitive,’ remarks Mr. W. Anderson
Smith, with reference to oysters,[297] ‘the creatures are to the
light above them; the shadow [of the boat] as it passes overhead is
instantaneously noted, and, snap! the lips are firmly closed.’
=Ocelli of Pecten.=--In _Pecten_ and _Spondylus_ the _ocelli_ are
remarkably large and prominent, shining like precious stones, and are
placed along the two edges of the mantle so as to receive the light
when the shell gapes (Fig. 93). In _Pecten opercularis_, _jacobaeus_,
and _maximus_ their number varies from 80 to 120. In _Spondylus
gaederopus_, a very inequivalve shell, 60 have been counted on the
right or fixed valve, and 90 on the left or upper valve. Each ocellus
is connected, by means of its optic nerve, with the large circumpalleal
nerve, and so with the branchial ganglion. They possess a cornea,
lens, choroidea, and optic nerve, and, according to Hickson,[298] bear
a considerable resemblance to the vertebrate type of eye. In spite
of this, the power of vision in these genera does not appear at all
superior to that of other Pelecypoda.
[Illustration: FIG. 93.--_Pecten opercularis_ L., showing the
_ocelli_ on the two edges of the mantle.]
[Illustration: FIG. 94.--Compound eyes (_c.e_) of _Arca barbata_
L.; _m.l_, mantle fold; _omm_, ommatidia. (After Patten.)]
According to the elaborate investigations of Patten, the ‘eyes’ in
_Arca_ occur upon the middle or ‘ophthalmic’ fold of the mantle-edge,
which is thickened at the end to admit of their reception. Along this
is ranged a row of dark brown spots of various sizes, which are larger
at the anterior and posterior ends of the mantle-edge, but smaller and
more numerous towards the middle. These brown spots, or ‘eyes,’ are
many of them compound, being made up of the fusion of a number of
ommatidia (from 10 to 80) into one large round eye, which is generally
elevated above the surface of the surrounding epithelium. Sometimes
these eyes themselves tend to fuse together. In one specimen of _Arca
Noae_, 133 of these faceted eyes were counted in one mantle border, and
102 in the other.
There can be little doubt that the development of these functional
eyes, or sensitive spots, in bivalve Mollusca, is due to special
needs. They appear to be entirely absent in fresh-water bivalves (with
the exception of _Dreissensia_, which is obviously a marine genus
recently become fresh-water), while they are most abundant in genera
living between tide marks (_Solen_, _Mya_, _Mactra_), and most highly
specialised in a genus that is, for a bivalve, of singularly active
habits (_Pecten_). Now genera living in sand between tide marks, as
the three above-mentioned genera are in the habit of doing, and also
protruding their siphons, and occasionally a considerable portion
of their shells, out of their burrow, are manifestly very much at
the mercy of their watchful enemies the gulls, and anything which
would enable them to apprehend the approach of their enemies would be
greatly to their advantage. Here, perhaps, lies the explanation of the
greater elaboration of these pigmented spots in littoral genera, as
compared with those inhabiting deeper water. _Pecten_, again, a genus
distinguished by great activity, which can ‘fly’ for considerable
distances in the water by flapping its valves together and expelling
the water from the apertures at either side of the hinge, may be
greatly assisted by its ocelli in directing its flight so as to escape
its enemies.
III. _Smell_
The sense of smell--touch at a distance, as Moquin-Tandon has called
it--is probably the most important sense which the Mollusca possess,
and is unquestionably far more valuable to them than that of sight.
Any one who has ever enjoyed the fun of hauling up lobster pots
will recollect that part of the contents was generally a plentiful
sprinkling of _Buccinum_, _Nassa_, and _Natica_, attracted by the smell
of the stinking piece of fish with which the trap was baited. According
to Mr. J. S. Gibbons,[299] _Bullia rhodostoma_ congregates in hundreds
on gigantic medusae which are stranded on the sandy bays near the
Cape of Good Hope. Dr. J. G. Jeffreys says[300] that quantities of the
common _Neptunea antiqua_ “are procured on the Cheshire coast by the
fishermen placing a dead dog on the sands at low-water mark during
spring tides. The bait is then completely covered with stones, which
are piled up like a cairn. On the next turn of the tide the heap of
stones is visited, and the whelks are found on the surface in great
numbers, having been apparently attracted by the smell of the bait,
but unable to get at it.” Mr. W. A. Lloyd kept specimens of _Nassa
reticulata_ in a tank in the sand, at the bottom of which they usually
remained buried. If a piece of meat of any kind were drawn over the
sand, the _Nassa_ would appear above the surface in a few minutes.
Half-picked beef or mutton bones, if placed in the tank, were covered
in a few minutes. In fact, no animal matter, whether living or dead,
could be introduced without the _Nassa_ smelling it, and coming up to
see what they could get.[301]
Any one can experiment for themselves on the olfactory powers of our
common snails or slugs. Moquin-Tandon records[302] two interesting
cases, one communicated to him by letter, the other occurring to
himself. His correspondent, a M. Parenteau, was one day walking along a
dusty high-road, when he noticed, near the middle of the road, an empty
bean-pod and two Arions eating it. Attributing the meeting of feeders
and food to mere chance, he was walking on, when he noticed a second
bean-pod, and, about two yards away from it, a third _Arion_, hurrying
straight towards it. When the _Arion_ had yet more than a yard to
traverse, M. Parenteau picked up the bean and put it in his pocket. The
_Arion_ stopped, raised its head, and turned in every direction, waving
its tentacles, but without advancing. M. Parenteau then carried the
bean to the other side of the road, and put it in a small hole behind a
piece of stone. The _Arion_, after a moment’s indecision, started off
straight for the bean. Again the position of the precious morsel was
changed, and again the _Arion_ made for it, this time without being
further tantalised. M. Moquin-Tandon noticed, one rainy day in the
botanical gardens at Toulouse, two _Limax maximus_ approaching a rotten
apple from different directions. He changed the position of the apple
several times, placing it at a sufficient distance, to be sure they
could not see it, but they always hit it off correctly, after raising
their heads and moving their long tentacles in every direction. It then
occurred to him to hold the apple in the air, some centimetres above
the head of the _Limax_. They perceived where it was, raised their
heads and lengthened their necks, endeavouring to find some solid body
on which to climb to their food.
Several of the land Mollusca have the power of exhaling a disagreeable
smell, _Hyalinia alliaria_ smelling strongly of garlic, and _Stenogyra
decollata_ of laudanum; but this need not be any argument for the sense
of smell in the creatures themselves.
=Position of Olfactory Organs in Pulmonata.=--Most authorities are
of opinion that the olfactory organs are situated in the tentacles.
Moquin-Tandon considered that in the Helicidae and Limacidae the sense
of smell is confined to the little knob or elevation at the end of
the longer tentacles, close to the eye. He found that when he cut off
these tentacles both in _Limax_ and _Arion_, the creatures were quite
unable to discover the whereabouts even of strongly-scented food. The
same author believed that in the Basommatophora the sense of smell
was present in the whole of the tentacle, which is covered with an
exceedingly sensitive ciliated epithelium. Lacaze-Duthiers, however,
places the olfactory sense in this group at the outer side of the base
of the tentacles, near to the eyes. Some authorities[303] deny that the
Helicidae have the olfactory organ at the tip of the tentacles, and
locate it in a pedal gland near the mouth, which contains conspicuous
sensitive cells. A _Helix_ whose tentacles had been removed manifested
its repulsion to the smell of spirits of turpentine, while another
_Helix_, which was unmutilated, did not object to the turpentine being
held between its tentacles. Altogether, then, the exact position of the
smell-organ in the Helicidae must be considered as not yet thoroughly
determined. Simroth holds that the sense of smell is distributed over
the whole soft integument, and is especially concentrated in the
feelers, and in the neighbourhood of the respiratory orifice.[304]
In nearly all marine Mollusca yet examined, the organ of smell or
_osphradium_ is in situation intimately connected with the breathing
organs, being generally placed near their base, with the object,
apparently, of testing the quality of the water before it passes over
the branchiae. It consists of a patch of the epithelium, modified in a
special manner, and connected by its own nerve with one of the visceral
ganglia.
An osphradium does not necessarily occur in all genera; for instance,
it has not been detected in _Fissurella_. It is most highly specialised
in the Conidae, and in the carnivorous Gasteropoda generally. In
_Buccinum undatum_, for instance, it is very large indeed, and, from
its plumed form, has sometimes been mistaken for an accessory branchia
(Fig. 95). In _Haliotis_ it is paired, one lying in close proximity to
each of the two branchiae, but in _Turbo_ it is single, corresponding
to the single branchia. In _Chiton_ there is an osphradium at the
base of each separate gill filament, making a total of twenty or more
on each side. Its position in _Physa_ and in _Cyclostoma_ will be
seen by reference to Figs. 103 and 104 (p. 205). In the Pelecypoda
the osphradia are paired, and lie adjacent to the posterior adductor
muscle, close to the hinder end of the axis of the branchiae. In the
Tetrabranchiate Cephalopoda there are two osphradia, placed between
the bases of the two pairs of gills. In the Dibranchiates on the other
hand, a groove above the eyes has been regarded as the seat of the
organ of smell. This groove contains sensory and ciliated cells, and
appears to be connected with a special nerve centre of its own, which
ultimately is derived from the cerebral ganglion.
[Illustration: FIG. 95.--_Buccinum undatum_ L., deprived of its
shell, showing the relative position of branchia (_br_) and
osphradium (_os_); _m_, mucous glands; _s_, siphon. The portion
of the mantle covering the osphradium has been removed.]
Scarcely any instances of the exercise of the sense of smell on the
part of bivalve Mollusca have been recorded. Something of the sort,
however, seems to have been present in a case related by Mr. R. L.
King.[305] A skull of a fox had been placed in a small ditch in order
to soak, and after a few days, when taken out, was found to be covered
with _Pisidium pusillum_ to the number of at least two hundred,
which had been probably attracted from the water in the immediate
neighbourhood by the smell of the decaying flesh.
IV. _Hearing_
Experiments made with a view to ascertain whether the Mollusca are
sensitive to noises have usually led to the conclusion that they
are deaf to very loud sounds. This is the more curious, because an
undoubted auditory apparatus has been discovered in a large number
of genera. In the case of an experiment, it is not easy to be sure
that the animal is not affected, at least in part, by the shock or
jar, rather than by the actual sound. In some experiments, however,
conducted at the Plymouth Marine Biological Laboratory, Mr. Bateson
found[306] that _Anomia_ could be made to shut its shell by smearing
the glass of the tank with the finger in such a way as to make a
creaking sound. It was evident that the cause of alarm was not the
jarring of the solid framework of the tank, for the same result
occurred when the object on which the _Anomia_ were fixed was suspended
in the water by a thread. It was found that the sound had to be of a
particular pitch to excite the attention of the mollusc.
As a rule the organ of hearing is nothing more than a small vesicle
or sac (the _otocyst_), filled with a fluid secretion, in which
are suspended one or usually more calcareous concretions known as
_otoliths_. By means of cilia, which connect with sense-cells, these
otoliths are given a peculiar movement or oscillation in the medium
in which they are suspended. The number of the otoliths varies in
different genera and species; there are several hundreds in _Arion_ and
_Limax_, about a hundred in _Helix pomatia_, _nemoralis_, _hispida_,
_arbustorum_, _rotundata_, _Succinea putris_, and _Limnaea stagnalis_;
about fifty in _Planorbis contortus_ and _Physa fontinalis_, only
one in _Cyclostoma elegans_. The number increases with age. In young
specimens of _Limn. stagnalis_ as few as ten, nine, and seven have been
noticed.[307]
The otocysts are always paired, and, in Gasteropoda, are placed close
to the pedal ganglia. The acoustic nerve, however, has been shown by
Lacaze-Duthiers to connect with the cerebral ganglia in certain cases.
The otocysts are never on the surface of the body and are rarely
connected with it by any passage or tube; it is probable therefore that
sound reaches them simply through the medium of the tissues.
In the _Pelecypoda_ the otocyst is similarly situated near the pedal
ganglion, and is probably (though this has not yet been proved)
similarly connected with the cerebral. There is only a single otolith.
Pelseneer finds[308] in Nuculidae alone a free communication between
the otocyst and the exterior. _Anodonta_ has been observed[309] to
withdraw its foot into the shell at the noise of an opening door, a
loud voice, or a shrill whistle, whether in a basin of water or lying
on a study table.
[Illustration: FIG. 96.--Illustrating the otocyst in =A=,
_Anodonta_, =B=, _Cyclas_; _ot_, otolith; _a_, _b_, _c_, _c´_,
cellular layers surrounding the chamber; _ci_, cilia on interior
walls of chamber: =C=, an otolith crushed. (After Simroth.)]
Delage extirpated the otocysts in certain Octopoda, and obtained some
unexpected results. He found that remarkable effects were produced upon
the animal’s powers of locomotion, so that it was unable to preserve
its proper balance in the water when in rapid motion, but its body
was forced to undergo a form of rotation more or less pronounced.
He concluded that the otocysts must possess, besides their auditory
functions, a power which stands in some relation to the proper
orientation of the body in locomotion, a power which is not wholly
supplied by sight and touch alone. The otocysts may thus regulate
locomotion by stimulating muscular acts which tend to keep the body in
the straight line during the process of movement.[310]
=The Foot=
One of the most characteristic organs of the Mollusca is the foot,
which, under one form or another, occurs throughout the whole phylum.
The foot is a thickening, on the ventral side, of a portion of the
integument of the animal, modified to serve different forms of
motion. It attains its maximum relative area in the Chitonidae, many
Nudibranchs, and the slugs generally, in nearly all of which there is
no portion of the body which is not subtended by the foot. Here too it
presents the form of a regular disc or ellipse, which is more or less
produced. In many cases, however, the foot becomes modified in such a
way that we are enabled to recognise well-marked anterior and posterior
portions, which have received the name of _propodium_ and _metapodium_
respectively, while the intervening central portion is termed the
_mesopodium_.
[Illustration: FIG. 97.--_Sigaretus laevigatus_ Lam., showing
excessive development of the propodium (_pr_) and metapodium
(_met_) in a mollusc living in sand (the shell, which covers only
the liver and adjacent parts, has been removed); _l_, liver;
_s.ap_, aperture of proboscis, here deflected from the median
line; _t_, _t_, tentacles. (After Quoy and Gaimard.)]
The propodium is most strongly developed in genera which crawl about in
wet sand, _e.g._ _Natica_, _Sigaretus_, _Oliva_, _Harpa_, _Scaphander_
(Figs. 97 and 98, and compare Fig. 91). In such cases it seems to serve
as a sort of fender or snow-plough, to push the sand away on both sides
of the path the animal is traversing. In some species of _Sigaretus_
the propodium becomes as it were banked up against the head and
proboscis, which are thus unnaturally elevated, or tend to disappear
altogether. _Bullia_ (Fig. 62), which crawls about rapidly on wet sand,
appears to attain its object by a wide extension of the foot on all
sides, and so slides over the sand instead of ploughing through it;
the little lappets at the end of the ‘tail’ probably serve as rudders.
In _Melampus_ and _Pedipes_ the propodium is marked off by a groove
across the ventral surface. When the animal is in motion it first
advances the propodium and then pulls the rest of the foot after it
with the looping gait of certain caterpillars. In many Cyclostomatidae
this groove, instead of being transverse, is longitudinal, and the
animal advances first the right and then the left segment of the foot,
which gives it a swaying motion from side to side.
Upon the metapodium lies the operculum, when it occurs. As a rule the
metapodium is not sharply marked off from the rest of the foot. In
_Strombus_ (Fig. 99) it becomes erected into a sort of hump or column,
on the top of which the operculum is situated.
[Illustration: FIG. 98.--_Oliva textilina_ Lam., showing how the
front part of the foot (_f_) is developed into a sort of fender,
the propodium (_pr_); _e_, _e_, eyes; _m.ap_, front appendage
of mantle; _m.ap´_, hinder appendage of mantle, folded into
the suture when the animal is at rest; _si_, siphon; _t_, _t_,
tentacles. (After Quoy and Gaimard.)]
The _epipodium_ is a prominent fold or border, which occurs upon the
upper edge of the foot in most Diotocardia. In _Haliotis_ it is of
considerable breadth, and is covered by a number of lobes which spring
from a moss-like prolongation of the skin. From the epipodium are
developed the lateral tentaculae of _Monodonta_ (Fig. 82, p. 178), and
of other sub-genera of the Trochidae.[311]
In the Opisthobranchiata the lateral edges of the foot (the
_parapodia_) are frequently produced into broad folds or wing-like
extensions, which in many cases tend to fold over the shell, and, in
conjunction with the mantle, eventually imbed it altogether. By the
wavy motion of the parapodia the animal is enabled to progress through
the water. The paired natatory lobes of the Pteropoda are simply the
parapodia of the Tectibranchs modified for swimming purposes.
[Illustration: FIG. 99.--_Strombus lentiginosus_ Lam., showing
the modified form of the foot (_f_): _e_, _e_, eyes on their
pedicels; _mp_, metapodium; _op_, operculum; _p_, penis; _pr_,
proboscis; _t_, _t_, tentacles. (After Quoy and Gaimard.)]
It is in the Heteropoda, Pteropoda, and most of all, the Cephalopoda,
groups which have, for the most part, exchanged a crawling for
a swimming life, that the modifications of the foot are most
considerable. In _Oxygyrus_ and _Atlanta_, for instance, the propodium
and metapodium are sharply distinguished from the mesopodium, and
no doubt have acquired, as a means of propulsion, the power of
separate movement, the animal swimming with these portions of the
foot uppermost. In _Carinaria_ and _Pterotrachea_ the metapodium has
probably become continuous with the long axis of the body, while
the so-called ‘foot’ with its sucker represents only the original
propodium. In the Cephalopoda the arms and funnel represent the
modified foot, the sides of which are prolonged into a number of very
long specialised tentaculae. In the adult Cephalopod some of the arms
have assumed a position in advance of the mouth, the latter being in
fact surrounded by a circle of arms. But in the Cephalopod embryo the
mouth opens as in the Gasteropoda, _i.e._ in advance of the arms, and
it is only gradually that it becomes encircled by them. Arms and funnel
alike are found to be innerved from the pedal ganglion.[312]
The pointed axe-shaped foot, which is characteristic of the majority of
Pelecypoda, is doubtless derived from a form more akin to the flattened
‘sole’ of the Gasteropoda. A foot with something of this disc-shaped
base actually occurs in some of the Nuculidae, the parapodia being
furnished with pleats which recall similar formations in other Orders
(Fig. 100). The principal modifications of the foot are due to its
employment as a burrowing organ. In genera which burrow but slightly
it is small and feebly developed, while in genera which habitually
excavate, it becomes the largest and strongest organ of the body. At
the same time it has a tendency to shift its position from the ventral
to the anterior margin, accompanied by a corresponding narrowing of
the shell, until it arrives at the position seen in Mollusca of the
shape of _Mya_, _Pholas_, and _Solen_. In sedentary or attached genera,
_e.g._ _Pecten_, _Chama_, _Ostrea_, the foot tends to become aborted.
[Illustration: FIG. 100.--_Yoldia limatula_ Say, Greenland,
showing the short plumed branchiae (_br_, _br_), the
gasteropodous foot (_f_), and the large labial palps (_l.p_,
_l.p_): =A=, as seen from the ventral margin; =B=, from the left
side, with the mantle turned back; _a.m_, position of anterior
adductor muscle; _i_, intestine; _l_, liver; _m_, _m_, mantle.]
The _byssus gland_, secreting a byssus of horny threads, is
characteristic of many Pelecypoda, and may be observed by any one
in the common mussel. It occurs in the larvae of many species which
do not possess a byssus in the adult stage. The pedal gland of many
Prosobranchiates, which secretes a tough and almost thread-like slime,
is possibly homologous with the byssus gland of bivalves.
=Nervous System=
The Mollusca possess a nervous system, which usually consists of a
number of nerve centres or _ganglia_, linked together by bands (the
_commissures_) and sending out thread-like nerves which ramify into the
various organs. The character of the nervous system varies greatly
in different groups, ranging as it does from a condition of extreme
complexity, in which the ganglia are numerous and the commissures
equally so, to that of considerable simplicity, in which ganglia are
almost entirely absent.
The most important ganglia are (1) the _cerebral_,[313] which are
always placed above or on either side of the mouth, and from which
proceed the nerves of the eyes and tentacles; (2) the _pedal_, which
in Gasteropoda are situated below the oesophagus, in Pelecypoda at the
base of the foot, and from which the nerves of the foot and sometimes
the acoustic nerve arise; (3) the _pleural_,[314] whose position varies
considerably, but is always below the oesophagus and slightly above
the pedal ganglia; these innervate the mantle, branchiae, heart, and
viscera generally.
=Gasteropoda.=--The simplest form of nerve system as thus understood
occurs in the Amphineura, and more particularly in the Chitons. Here
we find four longitudinal nerve-cords, parallel to one another for
nearly the whole length of the mollusc. The two exterior cords probably
represent the pleural, the two interior the pedal nervous system.
There being no head or tentacles, but simply a mouth at the anterior
end, the cerebral ganglia do not exist, but they are represented by
the curved ring formed by the massing together of the two nerve-cords
on each side. The only distinct ganglia are a pair of buccal ganglia
(which are developed on a pair of commissures which pass forward from
the cerebral mass and innervate the lips and buccal region), and a much
smaller group, the sublingual. The two pedal cords are united by a
number of transverse parallel connectives, which recall similar modes
of connexion in the Chaetopod worms and in Arthropoda.
This quadruple set of nerve-cords is characteristic of all the
Amphineura, but the absence of ganglia is most marked in the Chitons.
In _Proneomenia_ and _Neomenia_ there is a distinct cerebral
ganglion, formed by the massing of the two ganglia into one, while
in _Proneomenia_ the lateral cords are joined to the pedal, as well
as the pedal to one another, by connectives. In _Chaetoderma_ the
cerebral ganglia, though adjacent, are distinct, and both the pedal and
lateral cords connect directly with them, while there are no transverse
connectives.
The remaining three great divisions of Gasteropoda, namely, the
Prosobranchiata, Opisthobranchiata, and Pulmonata, may be regarded as
comprising two distinct types of nervous condition, according as the
loop formed by the two visceral nerve-cords is twisted over itself,
forming a figure of 8, or continues straight and uncrossed. In the
former case, we get the condition known as _streptoneurous_, in the
latter that as _euthyneurous_.[315] The _Euthyneura_ include the whole
of the Opisthobranchiata[316] and Pulmonata, the _Streptoneura_ all the
Prosobranchiata.
[Illustration: FIG. 101.--Nervous system of the Amphineura:
=A=, _Proneomenia_; =B=, _Neomenia_; =C=, _Chaetoderma_; =D=,
_Chiton_; _c_, cerebral ganglia; _l_, _l_, lateral cords; _pc_,
posterior commissure; _s_, sublingual commissure or ring, with
ganglia; _v_, _v_, pedal cords. (Alter Hubrecht.)]
The simplest form of nervous system in the euthyneurous Gasteropoda
occurs in the Opisthobranchiata. The cerebral, pleural, and pedal
ganglia tend to become concentrated in a ring-like form, united by
short commissures at the posterior end of the pharynx. The visceral
loop is in some cases long, and the two or three visceral ganglia are
then situated at its posterior extremity. The nervous system of the
Pulmonata is of a similar type, the visceral loop being often much
shorter, and tending to draw in towards the central group of ganglia.
The tentacular and optic nerves are, as usual, derived from the
cerebral ganglion, with which also the octocysts are probably connected
by rather long nerves. A pair of buccal ganglia innervate the buccal
mass, and are united by commissures with the right and left cerebral
ganglia. The osphradial nerve springs from one of the ganglia on the
visceral loop, the osphradium itself being situated (in _Limnaea_)
immediately above the pulmonary orifice and adjacent to the anus (Fig.
102). This massing of the ganglia is still better illustrated by the
accompanying figure of _Physa_ (Fig. 103), in which the animal is
represented as if transparent, so that the ganglia and nerves are seen
through the tissues.
[Illustration: FIG. 102.--I. Nervous system of _Limnaea
stagnalis_ L. The oesophagus has been cut and pulled forwards
through the nerve-collar, so as to expose the lower surface of
the buccal mass(dissected by F. B. Stead).
=B.M=, buccal mass.
=B.G=, buccal, =C.G=, cerebral, =Os.G=, osphradial, =Pe.G=, pedal
ganglia.
=Pl.G=, pleural ganglia.
=Op.N=, optic, =Os.N=, osphradial, =Te.N=, tentacular nerve.
=Ot=, otocyst; =V.L=, visceral loop.
=R=, rectum, dotted in to show its position relative to the
osphiadium.
II. Right side of the head of _Limnaea stagnalis_. The
overhanging flap of the mantle has been cut in the middle line,
and the right half twisted back, so as to expose the pulmonary
orifice, etc. The points =A= =A= on the mantle edge were
continuous before the mantle was cut; the line =BA= is part of
the free edge of the mantle.
=An=, anus; =F=, female generative orifice; =J=, portion of
jaw; =M=, male generative orifice under right tentacle; =Os=,
osphradium; =P.O=, pulmonary orifice.]
Of the streptoneurous Gasteropoda, the nervous system of _Fissurella_
and _Haliotis_ shows distinct points of similarity to that of the
Amphineura. The pedal nerves are united by transverse commissures
throughout their entire length, while a double commissure unites the
cerebral ganglia to the mass from which the pedal nerves proceed.
In the great majority of the Streptoneura the ganglia (except the
visceral) are more concentrated and the commissures are consequently
much shorter. The accompanying figure of _Cyclostoma_, in which
the animal is represented as in that of _Physa_ just described,
illustrates this grouping of the ganglia, the twist of the visceral
loop, and the position of the visceral ganglia at its posterior end.
[Illustration: FIG. 103.--Nervous system of _Physa acuta_
Drap., showing the massing of the ganglia at the hinder end of
the pharynx: _e_, _e_, eyes; _m_, mouth; _m.l_, _m.l_, mantle
lappets; _o.f_, female generative orifice; _o.m_, male generative
orifice; _os_, osphradium. (After Lacaze-Duthiers.)]
[Illustration: FIG. 104.--Example of a streptoneurous Gasteropod
(_Cyclostoma elegans_ Drap.): _c.g_, _c.g_, cerebral ganglia;
_e_, _e_, eyes; _os_, osphradium; _ot_, _ot_, otocysts; _p.g_,
_p.g_, pedal ganglia; _pl.g_, _pl.g_, pleural ganglia; _sp.g_,
supra-intestinal ganglion; _sb.g_, sub-intestinal ganglion;
_t.n_, tentacle nerve; _v.g_, visceral ganglion. (After
Lacaze-Duthiers.)]
=Scaphopoda.=--In the Scaphopoda the nervous system resembles that of
the Pelecypoda. The cerebral and pleural ganglia lie close together,
while the pedal ganglia are placed in the anterior part of the foot,
connected with the cerebral ganglia by long commissures; the visceral
loop is rather long, and the two visceral ganglia are adjacent to the
anus.
=Pelecypoda.=--The nervous system in the Pelecypoda is the simplest
type in which well-marked ganglionic centres occur. The ganglia are
few, symmetrically placed, and are usually at a considerable distance
apart. There are, as a rule, three distinct pairs of ganglia, the
cerebral (cerebro-pleural), pedal, and visceral. The cerebral are
formed by the fusion of the cerebral and pleural ganglia, which,
however, in some cases (Protobranchiata) continue distinct.[317] They
lie above or on each side of the mouth, united by a commissure of
varying length. Another pair of commissures unites them with the pedal
ganglia, which are placed at the base of the foot, and are usually
very close together, sometimes (as in _Anodonta_) becoming partially
fused. The length of these commissures depends upon the distance
between mouth and foot; thus they are very long in _Mya_ and _Modiola_,
and very short in _Pecten_. In cases where the foot is rudimentary or
becomes aborted through disuse (_e.g._ _Ostrea_), the pedal ganglia
may dwindle or disappear altogether. The visceral ganglia are on the
ventral side of the posterior adductor muscle, beneath the rectum,
and innervate the branchiae, osphradia, and the whole of the visceral
sac. A pair of cerebro-visceral commissures traverses the base of the
foot, surrounding it with a comparatively short loop (compare Fig. 106,
_c.v.c´_), while a long commissure, which runs round the entire edge
of the mantle, and supplies branching nerves to the mantle border and
siphons (Fig. 106, _c.v.c_), may also connect the visceral and cerebral
ganglia.
[Illustration: FIG. 105.--Nervous system of Pelecypoda: _A_,
_Teredo_; _B_, _Anodonta_; _C_, _Pecten_; _a_, _a_, cerebral
ganglia; _b_, pedal ganglia; _c_, visceral ganglia. (After
Gegenbaur.)]
=Cephalopoda.=--In the Cephalopoda the concentration of ganglia
attains its maximum, and may perhaps be regarded as approaching the
point at which a definite brain may be said to exist. Another point
of distinction is the formation of special small ganglia upon the
nerve-cords in different parts of the body. In the Tetrabranchiata
(_Nautilus_) the cerebral and pedal ganglia form a broad ring which
surrounds the oesophagus, the former giving out the optic nerves,
with their special optic ganglion, and a pair each of buccal and
pharyngeal ganglia, the latter the nerves of the arms and funnel. The
visceral loop is still present in the form of a separate band, which
innervates the branchiae, osphradia, and viscera generally, forming
a special genital ganglion in connexion with the reproductive organs.
The principal ganglia of the Dibranchiata are still more concentrated,
even the visceral loop being possibly united with the rest in forming
an unbroken mass in which scarcely any trace of commissures can be
detected. The pedal ganglion becomes separated into two portions,
one of which innervates the arms, the other the funnel. Two peculiar
ganglia (the stellate ganglia) supply a number of branching nerves to
the mantle.
[Illustration: FIG. 106.--Nervous system of _Cardium edule_
L.: _a.m_, anterior adductor muscle; _br_, branchiae; _br.n_,
branchial nerve; _c.g_, _c.g_, cerebral ganglia; _c.p.c_,
cerebro-pedal commissure; _c.v.c’_, cerebro-visceral commissure;
_c.v.c_, cerebro-visceral commissure of mantle; _l.p_, labial
palps: _m_, mouth; _p.g_, pedal ganglion; _p.m_, posterior
adductor muscle; _v.g_, visceral ganglion. (After Drost, × 3.)]
* * * * *
=E. L. Bouvier=, Système nerveux, morphologie générale et
classification des Gastéropodes prosobranches: Ann. Sc. Nat.
Zool. (7), iii. 1887, pp. 1–510.
=J. Brock=, Zur Neurologie der Prosobranchier: Zeit. wiss. Zool.
xlviii. 1889, pp. 67–83.
=O. Bütschli=, Bemerkungen über die wahrscheinliche Herleitung
der Asymmetrie der Gasteropoda, etc.: Morph. Jahrb. xii. 1886,
pp. 202–222.
=B. Haller=, Zur Kenntniss der Muriciden. I. Anatomie des
Nervensystems: Denksch. Math. Nat. Kl. Ak. Wien, xlv. 1882, pp.
87–106.
„ Untersuchungen über marine Rhipidoglossen. II. Textur des„
Centralnervensystems und seiner Hüllen: Morph. Jahrb. xi. 1885,
pp. 319–436.
=H. Grenadier=, Abhandlungen zur vergleichenden Anatomie des
Auges: Abh. Naturf. Gesell. Halle, xvi. 1884, pp. 207–256; xvii.
1886, pp. 1–64.
=A. P. Henchman=, The Origin and Development of the Central
Nervous System in _Limax maximus_: Bull. Mus. C. Z. Harv. xx.
1890, pp. 169–208.
=V. Hensen=, Ueber das Auge einiger Cephalophoren: Zeit. wiss.
Zool. xv. 1865, pp. 157–242.
=C. Hilger=, Beiträge zur Kenntniss des Gasteropodenauges:
Morph. Jahrb. x. 1885, pp. 352–371.
=Lacaze-Duthiers=, Otocystes ou Capsules auditives des
Mollusques (Gastéropodes): Arch. Zool. Exp. Gén. i. 1872, pp.
97–166.
„ „ Du système nerveux des Mollusques gastéropodes pulmonés
aquatiques: _ibid._ pp. 437–500.
=P. Pelseneer=, Recherches sur le système nerveux des
Ptéropodes: Arch. Biol. vii. 1887, pp. 93–130.
„ Sur la valeur morphologique des bras et la composition du
système nerveux central des Cephalopodes: Arch. Biol. viii.
1888, pp. 723–756.
=H. Simroth=, Ueber die Sinneswerkzeuge unserer einheimische
Weichthiere: Zeit. wiss. Zool. xxvi. 1876, pp. 227–348.
=J. W. Spengel=, Die Geruchsorgane und das Nervensystem der
Mollusken: Zeit. wiss. Zool. xxxv. 1881, pp. 333–383.
CHAPTER VIII
THE DIGESTIVE ORGANS, JAW, AND RADULA: EXCRETORY ORGANS
The digestive tract, or, as it is often termed, the alimentary canal or
gut, is a very important feature of the Mollusca. It may be regarded as
consisting of the following parts: (1) a _mouth_ or _oral aperture_:
(2) a _throat_ or _pharynx_; (3) an _oesophagus_, leading into (4) a
_stomach_, (5) an _intestine_ and _rectum_, ending in (6) an _anus_.
The primitive positions of mouth and anus were presumably at the
anterior and posterior ends of the animal, as in the Amphineura and
symmetrical Mollusca generally. But the modifications of original
molluscan symmetry, which have already been referred to (p. 154,
compare pp. 245, 246), have resulted in the anus becoming, in the great
majority of Gasteropoda, twisted forward, and occupying a position on
some point in the right side in dextral, and in the left in sinistral
species.
The process of digestion, as the food passes from one end of the tract
to the other, is performed by the aid of the secretions of various
glands, which open into the alimentary canal at different points in
its course. The principal of these are the _salivary glands_, situated
on the pharynx and oesophagus, and the _liver_, _biliary_ or _hepatic
gland_, connecting with the stomach. With these may be considered the
_anal_ and _ink-glands_, which, in certain genera, connect with the
terminal portion of the rectum.
1. The _mouth_ is generally, as in the common snail and periwinkle,
placed on the lower part of the head, and may be either a mere
aperture, circular or semicircular, in the head-mass, or, as is more
usual, may be carried on a blunt snout (compare Fig. 6, p. 10, and Fig.
68, p. 159), which is capable of varying degrees of protrusion. From
the retractile snout has doubtless been derived the long proboscis
which is so prominent a feature of many genera (compare Figs. 1, B, and
99), and in some (_e.g._ _Mitra_, _Dolium_) attains a length exceeding
that of the whole body. As a rule, Mollusca provided with a proboscis
are carnivorous, while those whose mouth is on the surface of the head
are Vegetable feeders, but this rule is by no means invariable. The
mouth is thickened round the aperture into ‘lips,’ which are often
extensile, and appear capable of closing upon and grasping the food.
In the Pelecypoda the mouth is furnished, on each side, with a pair
of special external lobes, the ‘labial palps,’ which appear to be of
a highly sensitive nature, and whose object it is to collect, and
possibly to taste, the food before it passes into the mouth.
2. The _Pharynx, Jaws, and Radula_.--Immediately behind the lips the
mouth opens into the muscular throat, pharynx, or buccal mass. The
pharynx of the Glossophora, _i.e._ of the Gasteropoda, Scaphopoda,
and Cephalopoda, is distinguished from that of the Pelecypoda,[318]
by the possession of two very characteristic organs for the rasping
or trituration of food before it reaches the oesophagus and stomach.
These are (_a_) the _jaw_ or _jaws_, and (_b_) the _radula_,[319]
_odontophore_, or _lingual ribbon_. The jaws bite the food, the
radula tears it up small before it passes into the stomach to undergo
digestion. The jaws are not set with teeth like our own; roughly
speaking, the best idea of the relations of the molluscan jaw and
radula may be obtained by imagining our own teeth removed from our jaws
and set in parallel rows along a greatly prolonged tongue.[320]
In nearly all land Pulmonata the jaw is single, and is placed behind
the upper lip. If a common _Helix aspersa_ be observed crawling up
the inside of a glass jar, or feeding on some succulent leaf, the
position and action of the jaw can be readily discerned. It shows
very black when the creature opens its mouth, and under its operation
the edge of a lettuce leaf shows a regular series of little curved
indentations, in shape not unlike the semicircular bites inflicted by
a schoolboy upon his bread and butter. The jaw of _Helix_ (Fig. 107,
B) is arched in shape, and is strengthened by a number of projecting
vertical ribs. That of _Limax_ (A) is straighter, and is slightly
striated, without vertical ribs. In _Bulimulus_ (C) the arch of the
jaw is very conspicuous, and the upper edges are always denticulated;
in _Orthalicus_ there is a central triangular plate with a number of
overlapping plates on either side; in _Succinea_ (E) there is a large
square accessory plate above the jaw proper. The form of the jaw
is peculiar not only to the genus but to the species as well. Thus
the jaw of _H. aspersa_ is specifically distinct from that of _H.
pomatia_, and that of _H. nemoralis_ is distinct from both. Wiegmann
has observed[321] that in young _Arion_, _Limax_, and _Helix_, the jaw
consists of two pieces, which coalesce by fusion in the adult, thus
indicating a stage of development in advance of the double jaw which
is found in most of the non-pulmonate Mollusca. In all fresh-water
Pulmonata there are two small accessory side plates besides the jaw
proper (Fig. 107, F).
[Illustration: FIG. 107.--Jaws of various Pulmonata: =A=, _Limax_
(_gagates_ Drap., Lancashire, × 15); =B=, _Helix_ (_acutissima_
Lam., Jamaica, × 15); =C=, _Bulimulus_ (_depictus_ Reeve,
Venezuela, × 20); =D=, _Achatina_ (_fulica_ Fér., Mauritius, ×
7); =E=, _Succinea_ (_elegans_ Riss., Aral District, × 30); =F=,
_Limnaea_ (_stagnalis_ L., Cambridge, × 30).]
Nearly all the non-carnivorous Prosobranchiata, land, fresh-water,
and marine alike, are provided with two large lateral jaws. Many of
these are sculptured with the most elaborate patterns, and appear to
be furnished with raised teeth, like a file. In the Nudibranchiata the
jaws are of great size and beauty of ornamentation (Fig. 109).
[Illustration: FIG. 108.--Jaws of =A=, _Triton australis_
Lam., Sydney; =B=, _Ampullaria fasciata_ Reeve, Demerara; =C=,
_Calliostoma punctulatum_ Mart., New Zealand; =D=, _Cyclophorus
atramentarius_ Sowb., Sanghir; all × 15.]
[Illustration: FIG. 109.--Jaws of =A=, _Chromodoris
gracilis_ Iher., × 15; =B=, _Scyllaea pelagica_ L., × 7; =C=,
_Pleurobranchus plumula_ Mont., × 10; =D=, _Pleurobranchaea
Meckelii_ Lam., × 5/2.]
The carnivorous genera, whether marine (_e.g._ _Conus_, _Murex_,
_Buccinum_, _Nassa_) or land (_e.g._ _Testacella_, _Glandina_,
_Streptaxis_, _Ennea_), are entirely destitute of jaws, the reason
probably being that in all these cases the teeth of the radula are
sufficiently powerful to do the work of tearing up the food without
the aid of a masticatory organ as well. Jaws are also wanting in the
Heteropoda, and in many of the Nudibranchiata and Tectibranchiata.
In the Cephalopoda the jaws, or ‘beaks,’ as they are called, are most
formidable weapons of attack. In shape they closely resemble the beaks
of a parrot, but the hook on the dorsal side of the mouth does not,
as in birds, close over the lower hook, but fits under it. Powerful
muscles govern these mandibles, which must operate with immense effect
upon their prey (Fig. 110).
[Illustration: FIG. 110.--Jaws of _Sepia_: =A=, _in situ_ within
the buccal mass, several of the arms having been cut away; =B=,
removed from the mouth and slightly enlarged.]
[Illustration: FIG. 111.--_Patella vulgata_ L., showing the
normal position of the radula, which is doubled back in a bow;
the shell has been removed, and the whole visceral mass is turned
forward, exposing the dorsal surface of the muscular foot:
_gr_, longitudinal groove on this surface; _i_, _i_, intestine;
_l_, liver; _m_, _m_, mantle edge; _mu_, muscles (cut through)
fastening the visceral mass to the upper sides of the foot; _ov_,
ovary; _r_, radula; _u.f_, upper or dorsal surface of the foot.]
_The Radula._[322]--When the food has passed beyond the operation
of the jaw, it comes within the province of the radula, the front
part of which perhaps co-operates to a certain extent with the jaw in
performing the biting process. The function of the radula as a whole
is to tear or scratch, not to bite; the food passes over it and is
carded small, the effect being very much the same as if, instead of
dragging a harrow over the surface of a field, we were to turn the
harrow points upwards, and then drag the field over the harrow.
The radula itself is a band or ribbon of varying length and breadth,
formed of chitin, generally almost transparent, sometimes beautifully
coloured, especially at the front end, with red or yellow.[323] It lies
enveloped in a kind of membrane, in the floor of the mouth and throat,
being quite flat in the forward part, but usually curving up so as to
line the sides of the throat farther back, and in some cases eventually
forming almost a tube. The upper surface, _i.e._ the surface over
which the food passes, is covered with teeth of the most varied shape,
size, number, and disposition, which are almost invariably arranged in
symmetrical rows. These teeth are attached to the cartilage on which
they work by muscles which serve to erect or depress them; probably
also the radula as a whole can be given a forward or backward motion,
so as to rasp or card the substances which pass over it.
The teeth on the front part of the radula are often much worn (Fig.
112), and probably fall away by degrees, their place being taken by
others successively pushed up from behind. At the extreme hinder end of
the radula the teeth are in a nascent condition, and there are often
as many as a dozen or more scarcely developed rows. Here, too, lie the
cells from which the teeth are originally formed.
The length and breadth of the radula vary greatly in different genera.
In _Littorina_ it is very narrow, and several times the length of the
whole animal. It is kept coiled away like a watch-spring at the back
of the throat, only a small proportion of the whole being in use. I
have counted as many as 480 rows in the common _Littorina littorea_.
In _Patella_ it is often longer than the shell itself, and if the
radula of a large specimen be freshly extracted and drawn across the
hand, the action of the hooks can be plainly felt. In _Aerope_, the
Turbinidae generally, and _Haliotis_ it is very large. In _Turritella_,
_Aporrhais_, _Cylichna_, _Struthiolaria_, and the Cephalopoda it
is small in proportion to the size of the animal. In the Pulmonata
generally it is very broad, the length not exceeding, as a rule, thrice
the breadth; in most other groups the breadth is inconsiderable, as
compared to the length.
The Radula is wanting in two families of Prosobranchiata, the Eulimidae
and Pyramidellidae, which are consequently grouped together as the
section Gymnoglossa. It is probable that in these cases the radula
has aborted through disuse, the animals having taken to a food
which does not require trituration. Thus several genera contained
in both these families are known to live parasitically upon various
animals--Holothurians, Echinoderms, etc.--nourishing themselves on
the juices of their host. In some cases, the development of a special
suctorial proboscis compensates for the loss of radula (see pp. 76–77).
In _Harpa_ there is no radula in the adult, though it is present in the
young form. No explanation of this fact has yet been given. It is also
absent in the Coralliophilidae, a family closely akin to _Purpura_,
but invariably parasitic on corals, and probably nourished by their
exudations. There is no radula in _Entoconcha_, an obscure form
parasitic on the blood-vessels of _Synapta_, or in _Neomenia_, a genus
of very low organisation, or in the Tethyidae, or sea-hares, or in one
or two other genera of Nudibranchiata.
[Illustration: FIG. 112.--Example of a front portion of a radula
(_Cantharus ringens_ Reeve, Panama), much worn by use. × 70.]
The number of teeth in the radula varies greatly. When the teeth are
very large, they are usually few in number, when small, they are
very numerous. In the carnivorous forms, as a rule, the teeth are
comparatively few and powerful, while in the phytophagous genera they
are many and small. Large hooked and sickle-shaped teeth, sometimes
furnished with barbs like an arrow-head, and poison-glands, are
characteristic of genera which feed on flesh; vegetable feeders, on
the contrary, have the teeth rounded, and blunter at the apex, or, if
long and narrow, so slender as to be of comparatively little effect.
Genera which are normally vegetarian, but which will, upon occasion,
eat flesh, _e.g._ _Limax_ and _Hyalinia_, exhibit a form of teeth
intermediate between these two extremes (see Fig. 140, A).
In _Chaetoderma_ there is but one tooth. In _Aeolis coronata_ there are
about 17, in _A. papillosa_ and _Elysia viridis_ about 19, in _Glaucus
atlanticus_ about 21, in _Fiona nobilis_ about 28. In the common
whelk (_Buccinum undatum_) there are from 220 to 250, in the common
periwinkle about 3500. As many as 8343 have been counted in _Limnaea
stagnalis_, about 15,000 in _Helix aspersa_ (that is, about 400,000
to the square inch), about 30,000 in _Limax maximus_, and as many as
40,000 in _Helix Ghiesbreghti_, a large species from Mexico; they are
very numerous also in _Nanina_, _Vitrina_, _Gadinia_, and _Actaeon_.
But _Umbrella_ stands far and away the first, as far as number of teeth
is concerned. In both _U. mediterranea_ and _U. indica_ they entirely
baffle calculation, possibly 750,000 may be somewhere near the truth.
The teeth on the radula are almost invariably disposed in a kind of
pattern, exactly like the longitudinal rows of colour in a piece of
ribbon, down the centre of which runs a narrow stripe, and every band
of colour on one side is repeated in the same relative position on
the other side. The middle tooth of each row--the rows being counted
across the radula, not longitudinally--is called the _central_ or
_rachidian_ tooth; the teeth next adjacent on each side are known as
the _laterals_, while the outermost are styled _uncini_ or _marginals_.
As a rule, the distinction between the laterals and marginals is fairly
well indicated, but in the Helicidae and some of the Nudibranchiata it
is not easy to perceive, and in these cases there is a very gradual
passage from one set to the other.
The central tooth is nearly always present. It is wanting in certain
groups of Opisthobranchiata, some of the carnivorous Pulmonata, and
in the Conidae and Terebridae, which have lost the laterals as well.
_Voluta_ has lost both laterals and marginals in most of the species,
and the same is the case with _Harpa_. In _Aeolis_, _Elysia_, and some
other Nudibranchiata the radula consists of a single central row. Other
peculiarities will be described below in their proper order.
The extreme importance of a study of the radula depends upon the fact,
that in each species, and _a fortiori_ in each genus and family, the
radula is characteristic. In closely allied species the differences
exhibited are naturally but slight, but in well-marked species the
differences are considerable. The radula, therefore, serves as a test
for the distinction of genera and species. For instance, in the four
known recent genera of the family Strombidae, _viz._ _Strombus_,
_Pteroceras_, _Rostellaria_, and _Terebellum_, the radula is of the
same general type throughout, but with distinct modifications for each
genus; and the same is true, though to a lesser extent, for all the
species hitherto examined in each of the genera. These facts are true
for all known genera, differences of the radula corresponding to and
emphasising those other differences which have caused genera to be
constituted. The radula therefore forms a _basis of classification_,
and it is found especially useful in this respect in dealing with the
largest class of all, the Gasteropoda, and particularly with the chief
section of this order, the Prosobranchiata. Thus we have--
{ (_a_) _Toxoglossa_
{ (_b_) _Rachiglossa_
{ _Monotocardia_ { (_c_) _Taenioglossa_
{ { (_d_) _Ptenoglossa_
=Prosobranchiata= { { (_e_) _Gymnoglossa_
{
{ _Diotocardia_ { (_f_) _Rhipidonlossa_
{ (_g_) _Docoglossa_[324]
(_a_) _Toxoglossa._--Only three families, Terebridae, Conidae, and
Cancellariidae, belong to this section. There is no central tooth,
and no laterals, the radula consisting simply of large marginals on
each side. In _Conus_ these are of great size, with a blunt base which
contains a poison-gland (see p. 66), the contents of which are carried
to the point by a duct. The point is always singly and sometimes doubly
barbed (Fig. 116). When extracted, the teeth resemble a small sheaf
of arrows (Figs. 113, 115). A remarkable form of radula, belonging to
_Spirotropis_ (a sub-genus of _Drillia_, one of the Conidae), enables
us to explain the true history of the radula in the Toxoglossa. Here
there are five teeth in a row, a central tooth, and one lateral and one
marginal on each side, the marginals being very similar in shape to the
characteristic shafts of the Conidae (Fig. 114). It is evident, then,
that the great mass of the Toxoglossa have lost both their central
and lateral teeth, and that those which remain are true uncini or
marginals. _Spirotropis_ appears to be the solitary survival of a group
retaining the primitive form of radula.
[Illustration: FIG. 113.--Radula of _Bela turricula_ Mont. × 70.]
[Illustration: FIG. 114.--Portion of radula of _Spirotropis
carinata_ Phil., Norway. × 70.]
[Illustration: FIG. 115.--Eight teeth from the radula of _Terebra
caerulescens_ Lam. × 60.]
The arrangement of teeth in all these sections is expressed by a
formula applicable to each transverse row of the series. The central
tooth, if present, is represented by 1, and the laterals and marginals,
according to their number, on each side of the central figure. Thus the
typical formula of the Toxoglossa is 1.0.0.0.1, the middle 0 standing
for the central tooth which is absent, and the 0 on each side of it for
the absent laterals; the 1 on each extreme represents the one uncinus
in each row. Thus the formula for _Spirotropis_, which has also one
lateral on each side and a rachidian or central tooth, is 1.1.1.1.1.
Often the formula is given thus: 1.0.0.0.1 / 30, 1.1.1.1.1 / 42, where
30 and 42 stand for the average number of _rows_ of teeth in _Conus_
and _Spirotropis_ respectively; the same is sometimes expressed thus:
1.0.0.0.1 × 30; 1.1.1.1.1 × 42.
[Illustration: FIG. 116.--A tooth from the radula of _Conus
imperialis_ L., S. Pacific, × 50, showing barb and poison duct.]
[Illustration: FIG. 117.--Portion of the radula of _Melongena
vespertilio_ Lam., Ceylon. × 30.]
[Illustration: FIG. 118.--Portion of the radula of _Eburna
japonica_ Sowb., China. × 30.]
[Illustration: FIG. 119.--Portion of the radula of _Murex regius_
Lam., Panama. × 60.]
(_b_) The _Rachiglossa_ comprise the 12 families Olividae, Harpidae,
Marginellidae, Volutidae, Mitridae, Fasciolariidae, Turbinellidae,
Buccinidae, Nassidae, Columbellidae, Muricidae, and Coralliophilidae.
Certainly most and probably all of these families are or have been
carnivorous, the Coralliophilidae being a degraded group which have
become parasitic on corals, and have lost their teeth in consequence.
The characteristics of the group are the possession of a central tooth
with from one cusp (_Boreofusus_) to about fourteen (_Bullia_), and
a single lateral more or less cuspidate, the outer cusp of all being
generally much the largest. Thus in _Melongena respertilio_ (Fig. 117)
the central tooth is tricuspid, the central cusp being the smallest,
while the laterals are bicuspid; in _Eburna japonica_ (Fig. 118) the
central tooth is 5-cusped, the two outer cusps being much the smallest.
The teeth, on the whole, are sharp and hooked, with a broad base and
formidable cutting edge. In the Olividae, _Turricula_, _Buccinopsis_,
and the Muricidae the laterals are unicuspid and somewhat degraded
(Fig. 119). In _Mitra_ and the Fasciolariidae they are very broad and
finely equally toothed like a comb (Figs. 120, 121). The whole group is
destitute of marginals.
[Illustration: FIG. 120.--Portion of the radula of _Imbricaria
marmorata_ Swains. × 80.]
[Illustration: FIG. 121.--Three rows of teeth from the radula of
_Fasciolaria trapezium_ Lam. × 40.]
[Illustration: FIG. 122.--Six teeth from the radula of _Cymbium
diadema_ Lam., Torres Strait. × 25.]