=A=, _Cantharus pagodus_ Reeve, Panama (nascent end), × 40; =A´=,
same radula, central and front portion; =B=, _Columbella varia_
Sowb., Panama, × 50.]
[Illustration: FIG. 124.--Three rows of the radula of _Sistrum
spectrum_ Reeve, Tonga, × 80. The laterals to the right are not
drawn in.]
Several remarkable peculiarities occur. _Harpa_ loses the radula
altogether in the adult. In the young it has lost only the laterals,
and consists of nothing but the central tooth. _Marginella_ has no
laterals; the central tooth is small and comb-shaped, with blunt cusps.
In _Voluta_ the laterals are generally lost, but in _Volutomitra_ and
one species of _Voluta_[325] they are retained. The central tooth
usually has three strong cusps, and is very thick and coloured a deep
red or orange (Fig. 122); in the sub-genus _Amoria_ it is unicuspid, in
shape rather like a spear-head with broadened wings; in _Volutolyria_
it is of a different type, with numerous unequal denticulations,
something like the laterals of _Mitra_ or _Fasciolaria_. Of the
Mitridae, _Cylindromitra_ has lost the laterals. Among the Buccinidae,
_Buccinopsis_ possesses a curiously degraded radula, the central
tooth having no cusps, but being reduced to a thin basal plate, while
the laterals are also weakened. This degradation from the type is a
remarkable feature among radulae, and appears to be characteristic,
sometimes of a whole family, _e.g._ the Columbellidae (Fig. 123, B),
sometimes of a genus, sometimes again of a single species. Thus in
_Cantharus_ (a sub-genus of _Buccinum_) the radula is typical in the
great majority of species, but in _C. pagodus_ Reeve, a large and
well-grown species, it is most remarkably degraded, both in the central
and lateral teeth (Fig. 123, A). This circumstance is the more singular
since _C. pagodus_ lives at Panama side by side with _C. ringeus_
and _C. insignis_, both of which have perfectly typical radulae. It
is probable that the nature of the food has something to do with the
phenomenon. Thus _Sistrum spectrum_ Reeve was found to possess a very
aberrant radula, not of the common muricoid type, but with very long
reed-like laterals. This singularity was a standing puzzle to the
present writer, until he was fortunate enough to discover that _S.
spectrum_, unlike all other species of _Sistrum_, lives exclusively on
a branching coral.
The dental formula for the _Rachiglossa_ is thus 1.1.1, except in those
cases where the laterals are absent, when it is 0.1.0.
[Illustration: Fig. 125.--Portion of the radula of _Cassis
sulcosa_ Born., × 40. The marginals to the right are not fully
drawn.]
(_c_) The _Taenioglossa_ comprise 46 families in all, of which the
most important are Tritonidae, Cassididae, Cypraeidae, Strombidae,
Cerithiidae, Turritellidae, Melaniidae, Littorinidae, Rissoidae,
Paludinidae, Ampullariidae, Cyclophoridae, Cyclostomatidae, and
Naticidae. The radula is characterised by a central tooth of very
variable form, the prevailing type being multicuspid, the central cusp
the largest, on a rather broad base; a single lateral, which is often a
broad plate, more or less cusped, and two uncini, rather narrow, with
single hooks, or slightly cusped. The accompanying figures of _Cassis_,
_Vermetus_, and _Cypraea_, and those of _Littorina_ and _Cyclophorus_
given on pp. 20, 21, are good examples of typical taenioglossate
radulae.
[Illustration: FIG. 126.--Four rows of teeth from the radula of
_Vermetus grandis_ Gray, Andamans. × 40.]
In _Homalogyra_ the radula is much degraded, the central tooth is
large and triangular on a broad base, the lateral is represented only
by a thin oblong plate, and the uncini are absent. In some species
of _Jeffreysia_ the uncini are said to be absent, while present in
others. _Lamellaria_ has lost both its uncini, but the radula of the
allied _Velutina_ is quite typical. A peculiar feature in this group
is the tendency of the marginals to increase in number. A stage in
this direction is perhaps seen in _Ovula_, _Pedicularia_, and the
_Cyclostomatidae_. Here the outermost of the two marginals is by far
the larger and broader, and is strongly pectinated on its upper edge;
in the _Cyclostomatidae_ the pectinations are rather superficial;
in _Ovula_ (where both marginals are pectinated) they are decidedly
deeper; in _Pedicularia_ they are deeper still, and make long slits
in the tooth, tending to subdivide it altogether. In _Turritella_ the
number of marginals is said to vary from none (in _T. acicula_) to
three (_T. triplicata_), but the fact wants confirmation. _Solarium_
is an aberrant form, possessing simply a number of long uncini, which
recall those of _Conus_ or _Pleurotoma_, and is therefore hard to
classify; the allied _Torinia_ has a radula which appears allied
to _Ovula_ or _Pedicularia_. In _Triforis_ the teeth are identical
throughout, very small, about 27 in a row, tricuspid on a square base,
cusps short.
The normal formula of the _Taenioglossa_ is 2.1.1.1.2; in _Lamellaria_,
1.1.1; in _Triforis_, 13.1.13, or thereabouts.
[Illustration: FIG. 127.--Two rows of the radula of _Cypraea
tigris_ L. × 30.]
[Illustration: FIG. 128.--Portion of the radula of _Ianthina
communis_ Lam. × 40.]
(_d_) _Ptenoglossa._--This section consists of two families only,
which certainly appear remarkably dissimilar in general habits
and appearance, _viz._, the Ianthinidae and Scalariidae. In all
probability their approximation is only provisional. The radula,
which in _Ianthina_ is very large, and in _Scalaria_ very small,
possesses an indefinite number of long hooked teeth, of which the
outermost are the largest. The central tooth, if present (it does not
occur in _Ianthina_), is the smallest in the series, and thus recalls
the arrangement in some of the carnivorous _Pulmonata_ (p. 232). In
_Ianthina_ the radula is formed of two large divisions, with a gap
between them down the middle.
The formula is ∞.1.∞ or ∞.O.∞ according as the central tooth in
_Scalaria_ is or is not reckoned to exist.
(_e_) _Gymnoglossa._--In the absence of both jaw and radula it is
not easy to classify the two families (Eulimidae and Pyramidellidae)
which are grouped under this section. Fischer regards them as modified
Ptenoglossa; one would think it more natural to approximate them to the
Taenioglossa.
[Illustration: FIG. 129.--Portion of the radula of _Margarita
umbilicalis_ Brod., Labrador. × 75 and 300.]
(_f_) _Rhipidoglossa._--This section consists of seventeen families,
the most important being the Helicinidae, Neritidae, Turbinidae,
Trochidae, Haliotidae, Pleurotomariidae, and Fissurellidae. The radula
is characterised by--
(1) The extraordinary development of the uncini, of which there are so
many that they are always reckoned as indefinitely numerous. They are
long, narrow, hooked, and often cusped at the top, and crowded together
like the ribs of a fan, those at the extreme edge not being set
straight in the row, but curving away backwards as they become smaller;
in _Solariella_ alone, where there are from five to ten, can they be
counted.
[Illustration: FIG. 130.--Portion of the radula of _Nerita
albicilla_ L., Andaman Is., with central tooth highly magnified:
_c_, _c_, the capituliform tooth. × 40.]
(2) The varying number of the laterals. The average number of these
is five on each side; in some cases (_Livona_) there are as many as
nine, in some (_Neritopsis_) only three. The lateral next to the uncini
(which is specially large in the Neritidae, and is then known as the
_capituliform_ tooth) is regarded by some authorities as the first
uncinus, by others as the sole representative of the laterals, the
teeth on the inner side of it being reckoned as multiplied central
teeth. According to this latter view, _Livona_ will have as many
as seventeen central teeth. Taking five as the average number of
‘laterals,’ we shall have the following different ways of constituting
the rhipidoglossate formula, the first being that to which preference
is given, _viz._:--
(1) ∞.5.1.5.∞, _i.e._ one central, five laterals, including
the ‘last lateral’ tooth.
(2) (∞.1).4.1.4.(1.∞), regarding the ‘last lateral’ as first
uncinus, but specialising it by a number.
(3) ∞.1.(4.1.4).1.∞, regarding the ‘last lateral’ as the only
lateral.
In the Neritidae and the derived fresh-water genera (_Neritina_,
_Navicella_) the first lateral, as well as the capituliform
tooth, is very large, and in shape rather like the blade bone of
a shoulder of mutton; the intervening laterals are very small. In
_Neritopsis_ (a degraded form) the central tooth and first lateral
are entirely wanting. In the neritiform land-shells (_Helicina_,
_Proserpina_) the first lateral is no larger than the others, while
the capituliform tooth is enormous. _Hydrocena_ is a very aberrant
and apparently degraded form; the laterals between the first and the
capituliform tooth are all wanting. In _Haliotis_, _Scissurella_,
and _Pleurotomaria_ the five laterals are of fairly equal size; in
_Fissurella_ we again meet with a large capituliform tooth, with very
small laterals.
(_g_) The _Docoglossa_ are in direct contrast with the Rhipidoglossa
in possessing few and strong teeth, instead of many and weak. There
are only three families, Acmaeidae, Patellidae, and Lepetidae. In some
of the Acmaeidae there are not more than two teeth in a row, while in
no species are there more than twelve. The radula is, however, very
long; there are as many as 180 rows in _Patella vulgata_. The teeth
are thick, generally of a very deep red horn colour, rather opaque.
The cartilage in which they are set is remarkably thick, and in some
species whose teeth are very few a considerable portion of this
cartilage is left quite bare.
[Illustration: FIG. 131.--Portion of the radula of _Patella
cretacea_ Reeve, seen in half profile. × 40.]
Although the teeth are so few, the arrangement is by no means simple.
The special feature of the group is the multiplication of identical
centrals. Of these there are two in _Acmaea_, and four, as a rule,
in _Patella_. Thus in these two genera there is seldom an absolutely
_central_ tooth. Either laterals or marginals are liable to be lost,
but there are never more than two of either in _Acmaea_, and never
more than two laterals and three marginals in _Patella_. Thus the
formula varies from 0.0.(1 + 0 + 1).0.0 in _Pectinodonta_, 2.2.(1 + 0
+ 1).2.2 in _Collisellina_ (both Acmaeidae), to 3.2.(1 + 0 + 1).2.3
in _Patinella_, and 3.1.(2 + 0 + 2).1.3 in _Patella_ proper. In the
Lepetidae there is an absolutely central tooth, which appears to be
made up of the coalescence of several teeth, no laterals, and about two
marginals; formula, 2.0.1.0.2.
[Illustration: FIG. 132.--Two rows of the radula of
_Pterotrachea mutica_ Les., Naples. × 60.]
The radula of the _Heteropoda_ is quite characteristic, and shows
no sign of affinity with any other Prosobranchiate. The central tooth
is large, broad, tricuspid, and denticulated on a broad base; the
single lateral is strong, often bicuspid; the two marginals simple,
long, falciform; formula, 2.1.1.1.2 (Fig. 132).
[Illustration: FIG. 133.--=A=, Portion of the radula of _Chiton_
(_Acanthopleura_) _spiniger_] Sowb., Andamans, × 30; =B=, portion
of the radula of _Dentalium entalis_ L., Clyde, × 50.]
=Amphineura.=--(_a_) _Polyplacophora._--The radula of the _Chitonidae_
is quite unique. It resembles that of the _Docoglossa_ in being very
long, and composed of thick and dark horn-coloured teeth. The number of
teeth, however, is considerably greater, amounting almost invariably
to seventeen in each row. There are three rather small central teeth,
the two outer of these being similar; next comes a very large lateral
(the _major_ lateral), usually tricuspid, which is followed by two much
smaller laterals, which are scarcely more than accessory plates; then
a very large and arched marginal (the _major_ uncinus), at the outer
side of which are three accessory plates. Some consider there is only
one central tooth, and count the two small teeth on each side of it as
laterals.
Thus the formula is either (3 + 1).(2 + 1).3.(1 + 2).(1 + 3) or (3 +
1).(2 + 1 + 1).1.(1 + 1 + 2).(1 + 3).
(_b_) _Aplacophora._--Of this rather obscure order, _Chaetoderma_ has a
single strong central tooth, _Neomenia_ has no radula, _Proneomenia_
and _Lepidomenia_ have about twenty falciform teeth, much larger at one
end of the radula than the other; formula, 0.1.0.
=Opisthobranchiata.=--The radula of the Opisthobranchiata is
exceedingly variable in shape, size, and number and character of
teeth. Not only do allied families differ greatly from one another,
but allied genera often possess radulae widely distinct in plan.
Thus, among the Polyceridae, _Goniodoris_ has no central tooth, one
large lateral and one marginal (form. 1.1.0.1.1); _Doridunculus_ the
same, with five marginals (form. 5.1.0.1.5); _Lamellidoris_ one each
of median, laterals, and marginals (1.1.1.1.1); _Idalia_, _Ancula_,
and _Thecacera_ the same as _Goniodoris_; _Crimora_ several each of
laterals and marginals. Even species of the same genus may differ; thus
the formula for _Aeolis papillosa_ is 0.1.0, but for _Ae. Landsbergi_
1.1.1; for _Philine aperta_ 1.0.1, but for _Philine pruinosa_ 6.0.6.
[Illustration: FIG. 134.--Two teeth from the radula of _Aeolis
papillosa_ L. × 55.]
It must not be forgotten, however, that a simple repetition of the same
tooth, whether lateral or marginal, does not necessarily constitute
an important characteristic, nor does the presence or absence of a
central tooth. In most of the cases mentioned above, the difference in
the number of laterals and marginals is due to the multiplication of
identical forms, while the central tooth, when present, is often a mere
plate or narrow block without cusps, whose presence or absence makes
little difference to the character of the radula as a whole.
There appear to be three well-marked types of radula among the
Opisthobranchiata.
(_a_) Radula with a single strong central tooth, rows few. This form
is characteristic of the Aeolididae, Fionidae, Glaucidae, Dotoidae,
Hermaeidae, Elysiidae (Fig. 135), and Limapontiidae. In the Aeolididae
it is sometimes accompanied by a single lateral. The same type occurs
in _Oxynoe_, and in _Lobiger_ (= _Lophocercus_).
(_b_) Radula with the first lateral very strongly developed. This type
may take the form of (1) a single lateral, no central or marginals,
_e.g._ _Onchidoris_, _Scaphander_ (Fig. 137, A), _Philine_ (certain
species), _Ringicula_, or (2) first lateral strongly developed, and
repeated in succeeding laterals (2–6) on a smaller scale, _e.g._
_Philine_ (certain species). A few marginals are sometimes added,
_e.g._ in _Polycera_, _Lamellidoris_ (where there is a degraded central
tooth, Fig. 137, B), _Idalia_, and _Ancula_.
[Illustration: FIG. 135.--Radula of _Elysia viridis_ Mont. × 40.
Type (_a_).]
[Illustration: FIG. 136.--Portion of the radula of _Gadinia
peruviana_ Sowb., Chili. × 250. Type (_c_).]
(_c_) Radula with an indefinite number of marginals, laterals (if
present) merging into marginals, central tooth present or absent,
inconspicuous, teeth all very small. This type of radula, among the
Nudibranchiata, is characteristic of certain sub-genera of _Doris_
(_e.g._ _Chromodoris_, _Aphelodoris_, _Casella_, _Centrodoris_), of
_Hypobranchiaea_ and _Pleurophyllidia_; among the Tectibranchiata,
of _Actaeon_, many of the Bullidae, _Aplustrum_, the Aplysiidae,
_Pleurobranchus_, _Umbrella_ and _Gadinia_ (Figs. 136 and 137, C).
In the _Pteropoda_ there are two types of radula. The Gymnosomata,
which are in the main carnivorous, possess a radula with a varying
number (4–12) of sickle-shaped marginals, central tooth present or
absent. In the Thecosomata, which feed on a vegetable diet, there are
never more than three teeth, a central and a marginal on each side;
teeth more or less cusped on a square base.
=Pulmonata.=--The radula of the Testacellidae, or carnivorous land
Mollusca, is large, and consists of strong sickle-shaped teeth with
very sharp points, arranged in rows with or without a central tooth,
in such a way that the largest teeth are often on the outside, and the
smallest on the inside of the row (as in _Rhytida_, Fig. 139). The
number and size of the teeth vary. In _Testacella_ and _Glandina_,
they are numerous, consisting of from 30 to 70 in a row, with about
50 rows, the size throughout being fairly uniform. In _Aerope_ they
are exceedingly large, and only eight in a row, the outermost marginal
being probably the largest single tooth in the whole of the Mollusca.
The central tooth is always obscure, being, when present, simply a
weaker form of the weakest lateral; in genera with only a few teeth in
a row it is generally absent altogether.
[Illustration: FIG. 137.--Portions of the radula of
Opisthobranchiata, illustrating types (_b_) and (_c_); =A=,
_Scaphander lignarius_ L.; =A´=, one of the teeth seen from the
other side, × 40; =B=, _Lamellidoris bilamellata_ L., Torbay, ×
60; =C=, _Hydatina physis_ L., E. Indies, × 75.]
The first family of jaw-bearing snails, the Selenitidae, is distinctly
intermediate. The possession of a jaw relates it to the main body of
Helicidae, but the jaw is not strong, while the teeth are still, with
the exception of the central, thoroughly Testacellidan. The central
tooth is quite rudimentary, but it is something more than a mere
weak reproduction of the marginals. There are no true laterals. The
Limacidae show a further stage in the transition. Here the central
tooth has a definite shape of its own, tricuspid on a broad base, which
is more or less repeated in the first laterals; these, as they approach
the marginals, gradually change in form, until the outer marginals
are again thoroughly Testacellidan.[326] This is the general form
of radula, varied more or less in different genera, which occurs in
_Nanina_, _Helicarion_, _Limax_, _Parmacella_, and all the sub-genera
of _Zonites_. It is certain that some, and probable that all of these
genera will, on occasion, eat flesh, although their usual food appears
to be vegetable. The jaw is more powerful than in the Selenitidae, but
never so large or so strongly ribbed as in _Helix_ proper.
[Illustration: FIG. 138.--Portion of the radula of _Glandina
truncata_ Gmel. × 40.]
[Illustration: FIG. 139.--Portion of the radula of _Rhytida
Kraussii_ Pfr., S. Africa. × 25.]
When we reach the Helicidae, we arrive at a type of radula
in which the aculeate form of tooth--so characteristic of the
Agnatha--disappears even in the marginals, and is replaced by teeth
with a more or less quadrate base; the laterals, which are always
present, are intermediate in form between the central and the
marginals, and insensibly pass into the latter. In size and number
of cusps the first few laterals resemble the central tooth; in the
extreme marginals the cusps often become irregular or evanescent. As
a rule, the teeth are set squarely in the rows, with the exception of
the extreme marginals, which tend to slope away on either side. In
some Helicidae there is a slight approximation to the Zonitidae in the
elongation of the first marginals.
The above is the type of radula occurring in the great family
Helicidae, which includes not only _Helix_ proper, with several
thousand species, but also _Arion_, _Bulimus_, _Ariolimax_, and other
genera. The jaw is almost always strongly transversely ribbed.
In the _Orthalicidae_ (Fig. 140, C) the teeth of the radula, instead of
being in straight rows, slope back at an angle of about 45 degrees from
the central tooth. The central and laterals are very similar, with an
obtuse cusp on rather a long stem; the marginals become bicuspid.
In the _Bulimulidae_, which include the important genera _Placostylus_,
_Amphidromus_, _Partula_, _Amphibulimus_, and all the groups of South
American _Bulimulus_, the jaw is very characteristic, being thin,
arched, and denticulated at the edges, as if formed of numerous
narrow folds overlapping one another. The radula is like that of the
Helicidae, but the inner cusp of the laterals is usually lengthened and
incurved. In _Partula_ the separation between laterals and marginals is
very strongly marked.
The remaining families of Pulmonata must be more briefly described. In
the _Cylindrellidae_ there are three distinct types of radula: (_a_)
Central tooth a narrow plate, laterals all very curiously incurved
with a blunt cusp, no marginals (Fig. 140, D); (_b_) radula long and
narrow, central tooth as in (_a_), two laterals, and about eight small
marginals; (_c_) much more helicidan in type, central and laterals
obtusely unicuspid, marginals quite helicidan. Type (_c_) is restricted
to Central America, types (_a_) and (_b_) are West Indian.
_Pupidae_: Radula long and narrow; teeth of the helicidan type,
centrals and laterals tricuspid on a quadrate base, marginals very
small, cusps irregular and evanescent. This type includes _Anostoma_,
_Odontostomus_, _Buliminus_, _Vertigo_, _Strophia_, _Holospira_,
_Clausilia_, and _Balea_.
_Stenogyridae_, including _Achatina_, _Stenogyra_, and all its
sub-genera: Central tooth small and narrow, laterals much larger,
tricuspid, central cusp long, marginals similar, but smaller.
_Achatinellidae_: Two types occur; (_a_) teeth in very oblique rows,
central, laterals, and marginals all of the same type, base narrow,
head rather broad, with numerous small denticles (_Achatinella_ proper,
with _Auriculella_ and _Tornatellina_, Fig. 140, E); (_b_) central
tooth small and narrow, laterals bicuspid, marginals as in _Helix_
(_Amastra_ and _Carelia_).
[Illustration: FIG. 140.--Portions of the radula of =A=,
_Hyalinia nitidula_ Drap., Yorkshire, with central tooth, first
lateral, and a marginal very highly magnified; =B=, _Helix
pomatia_ L., Kent, showing central tooth, laterals, and one
extreme marginal, the two former also highly magnified; =C=,
_Orthalicus undatus_ Brug., Trinidad, with three laterals highly
magnified; =D=, _Cylindrella rosea_ Pfr., Jamaica, central tooth
and laterals, the same very highly magnified; =E=, _Achatinella
vulpina_ Fér., Oahu, central tooth (_c_) and laterals, the same
highly magnified.]
_Succineidae_: Central and laterals helicidan, bi- or tricuspid on a
quadrate plate, marginals denticulate on a narrow base; jaw with an
accessory oblong plate.
_Janellidae_: Central tooth very small, laterals and marginals like
_Achatinellidae_ (_a_).
_Vaginulidae_: Central, laterals, and marginals unicuspid throughout,
on same plan.
_Onchidiidae_: Rows oblique at the centre, straight near the edges;
central strong, tricuspid; laterals and marginals very long, falciform,
arched, unicuspid.
_Auriculidae_: Teeth very small; central narrow, tricuspid on rather a
broad base; laterals and marginals obscurely tricuspid on a base like
_Succinea_.
_Limnaeidae_: Jaw composed of one upper and two lateral pieces; central
and lateral teeth resembling those of Helicidae; marginals much
pectinated and serriform (Fig. 141, A). In _Ancylus_ proper the teeth
are of a very different type, base narrow, head rather blunt, with
no sharp cusps, teeth similar throughout, except that the marginals
become somewhat pectinated (Fig. 141, B); another type more resembles
_Limnaea_.
[Illustration: FIG. 141.--Portions of the radula of =A=, _Limnaea
stagnalis_ L., with the central tooth and two first laterals,
and two of the marginals, very highly, magnified; =B=, _Ancylus
fluviatilis_ Müll., with two of the marginals very highly
magnified; =C=, _Physa fontinalis_ L., with central tooth and two
of the marginals very highly magnified.]
_Physidae_: Jaw simple, but with a fibrous growth at its upper edge,
which may represent an accessory plate; radula with very oblique
rows, central tooth denticulate, laterals and marginals serriform,
comb-like, with a wing-like appendage at the superior outer edge (Fig.
141, C).
_Chilinidae_: Central tooth small, cusped on an excavated triangular
base, marginals five-cusped, with a projection as in _Physa_, laterals
comb-like, serrations not deep.
_Amphibolidae_: Central tooth five-cusped on a broad base, central
cusp very large; two laterals only, the first very small, thorn-like,
the second like the central tooth, but three-cusped; laterals simple,
sabre-shaped.
=Scaphopoda.=--In the single family (_Dentaliidae_) the radula is
large, and quite unlike that of any other group. The central tooth is a
simple broad plate; the single lateral is strong, arched, and slightly
cusped; the marginal a very large quadrangular plate, quite simple;
formula, 1.1.1.1.1 (Fig. 133, B).
=Cephalopoda.=--The radula of the Cephalopoda presents no special
feature of interest. Perhaps the most remarkable fact about it is its
singular uniformity of structure throughout a large number of genera.
It is always very small, as compared with the size of the animal, most
of the work being done by the powerful jaws, while the digestive powers
of the stomach are very considerable.
The general type of structure is a central tooth, a very few laterals,
and an occasional marginal or two; teeth of very uniform size and shape
throughout. In the Dibranchiata, marginals are entirely absent, their
place being always taken, in the Octopoda, by an accessory plate of
varying shape and size. This plate is generally absent in the Decapoda.
The central tooth is, in the Octopoda, very strong and characteristic;
in _Eledone_ and _Octopus_ it is five-cusped, central cusp strong; in
_Argonauta_ unicuspid, in _Tremoctopus_ tricuspid. The laterals are
always three in number, the innermost lateral having a tendency to
assume the form of the central. In _Sepia_ the two inner laterals are
exact reproductions of the central tooth; in _Eledone_, _Sepiola_,
_Loligo_, and _Sepia_, the third lateral is falciform and much the
largest.
[Illustration: FIG. 142.--Portion of the radula of _Octopus
tetracirrhus_ D. Ch., Naples, × 20.]
In _Nautilus_, the only living representative of the Tetrabranchiata,
there are two sickle-shaped marginals on each side, each of which has
a small accessory plate at the base. The two laterals and the central
tooth are small, very similar to one another, unicuspid on a square
base.
[Illustration: FIG. 143.--Alimentary canal of _Helix aspersa_
L.: _a_, anus; _b.d_, _b.d´_, right and left biliary ducts;
_b.m_, buccal mass; _c_, crop; _h.g_, hermaphrodite gland; _i_,
intestine; _i.o_, opening of same from stomach (pyloric orifice);
_l_, _l´_, right and left lobes of liver; _m_, mouth; _oe_,
oesophagus; _r_, rectum; _s.d_, salivary duct; _s.g_, salivary
gland; _st_, stomach; _t_, left tentacle. (After Howes and
Marshall, slightly modified.)]
_Salivary glands_ are found in most Glossophora. They occur in one or
two pairs on each side of the pharynx and oesophagus, the duct usually
leading forwards and opening into the anterior part of the pharynx
(see Figs. 143, 144). They are exceptionally large in the carnivorous
Gasteropoda. In certain genera, _e.g._ _Murex_, _Dolium_, _Cassis_,
_Pleurobranchus_, the secretions of these glands are found to contain
a considerable proportion (sometimes as much as 4·25 per cent) of
free sulphuric acid. This fact was first noticed by Troschel, who,
while handling a _Dolium galea_ at Messina, saw the creature spit a
jet of saliva upon a marble slab, which immediately produced a brisk
effervescence. A number of the genera thus provided bore through the
shells of other Mollusca and of Echinoderms, to prey upon their soft
tissues, and it is possible that the acid assists in the piercing of
the shell by converting the hard carbonate of lime into sulphate of
lime, which can easily be removed by the action of the radula.[327] In
the majority of the Cephalopoda there are two pairs of salivary glands,
one lying on each side of the mouth, the other on the middle of the
oesophagus.
[Illustration: FIG. 144.--Alimentary canal, etc., of _Sepia
officinalis_ L.: _a_, anus; _b.d_, one of the biliary ducts;
_b.m_, buccal mass; _c_, coecum; _i_, ink-sac; _i.d_, duct of
same; _j_, jaws; _l.l_, lobes of the liver; _oe_, oesophagus;
_p_, pancreatic coeca; _r_, rectum; _s.g_, salivary glands; _st_,
stomach. (From a specimen in the British Museum.)]
[Illustration: FIG. 145.--Gizzard of _Scaphander lignarius_
L.: =A=, showing position with regard to oesophagus (_oe_) and
intestine (_i_), the latter being full of comminuted fragments
of food; _p_, left plate; _p´_, right plate; _p.ac_, accessory
plate; =B=, the plates as seen from the front, with the
enveloping membranes removed, lettering as in =A=. Natural size.]
[Illustration: FIG. 146.--Section of the stomach of _Melongena_,
showing the gastric plates (_g.p_, _g.p_,) for the trituration of
food; _b.d_, biliary duct; _g.g_, genital gland; _i_, intestine;
_l_, liver; _oe_, oesophagus; _st_, stomach. (After Vanstone.)]
3. _The Oesophagus._--That part of the alimentary canal which lies
between the pharynx and the stomach (in Pelecypoda between the mouth
and stomach) is known as the oesophagus. Its exact limits are not easy
to define, since in many cases the tube widens so gradually, while the
muscular structure of its walls changes so slowly that it is difficult
to say where oesophagus ends and stomach begins. As a rule, the
oesophagus is fairly simple in structure, and consists of a straight
and narrow tube. In the Pulmonata and Opisthobranchiata it often widens
out into a ‘crop,’ which appears to serve the purpose of retaining a
quantity of masticated food before it passes on to the stomach. In
_Octopus_ and _Patella_ the crop takes the form of a lobular coecum.
In the carnivorous Mollusca the oesophagus becomes complicated by
the existence of a varying number of glands, by the action of which
digestion appears to begin in some cases before the food reaches the
stomach proper.
4. _The Stomach._--At the posterior end of the oesophagus lies the
muscular pouch known as the stomach, in which the digestion of the
food is principally performed. This organ may be, as in _Limax_, no
more than a dilatation of the alimentary canal, or it may, as is
usually the case, take the form of a well-marked bag or pocket. The two
orifices of the stomach are not always situated at opposite ends; when
the stomach itself is a simple enlargement of the wall of one side of
the alimentary canal, the cardiac or entering orifice often becomes
approximated to the orifice of exit (pyloric orifice).
The walls of the stomach itself are usually thickened and strengthened
by constrictor muscles. In some Nudibranchs (_Scyllaea_, _Bornella_)
they are lined on the inside with chitinous teeth. In _Cyclostoma_, and
some _Bithynia_, _Strombus_, and _Trochus_ there is a free chitinous
stylet within the stomach.[328] In _Melongena_ (Fig. 146) the posterior
end of the oesophagus is provided with a number of hard plate-like
ridges, while the stomach is lined with a double row of cuticular
knobs, which are movable on their bases of attachment, and serve the
purpose of triturating food.[329] _Aplysia_ has several hard plates,
set with knobs and spines, and similar organs occur in the Pteropoda.
But the most formidable organ for the crushing of food is possessed by
the Bullidae, and particularly by _Scaphander_ (Fig. 145). Here there
is a strong gizzard, consisting of several plates connected by powerful
cartilages, which crush the shells, which are swallowed whole.
Into the stomach, or into the adjacent portions of the digestive tract,
open the ducts which connect with the so-called _liver_. The functions
of this important organ have not yet been thoroughly worked out. The
liver is a lobe-shaped gland of a brown-gray or light red colour, which
in the spirally-shelled families usually occupies the greater part of
the spire. In the Cephalopoda, the two ducts of the liver are covered
by appendages which are usually known as the pancreatic coeca; the
biliary duct, instead of leading directly into the stomach, passes
into a very large coecum (see Fig. 144) or expansion of the same,
which serves as a reservoir for the biliary secretions. At the point
of connexion between the coecum and stomach is found a valve, which
opens for the issue of the biliary products into the stomach, but
closes against the entry of food into the coecum. In most Gasteropoda
the liver consists of two distinct lobes, between which are embedded
the stomach and part of the intestine. In many Nudibranchiata the liver
becomes ‘diffused’ or broken up into a number of small diverticula or
glands connecting with the stomach and intestine. The so-called cerata
or dorsal lobes in the Aeolididae are in effect an external liver,
the removal of which to the outside of the body gives the creature
additional stomach-room.
=The Hyaline Stylet.=--In the great majority of bivalves the intestine
is provided with a blind sac, or coecum of varying length. Within
this is usually lodged a long cylindrical body known as the _hyaline_
or _crystalline stylet_. In a well-developed _Mytilus edulis_ it is
over an inch in length, and in _Mya arenaria_ between two and three
inches. The bladder-like skin of the stylet, as well as its gelatinoid
substance, are perfectly transparent. In the Unionidae there is no
blind sac, and the stylet, when present, is in the intestine itself. It
is said to be present or absent indifferently in certain species.
The actual function performed by the hyaline stylet is at present a
matter of conjecture. Haseloff’s experiments on _Mytilus edulis_ tend
to confirm the suggestion of Möbius, that the structure represents
a reserve of food material, not specially secreted, but a chemical
modification of surplus food. He found that under natural conditions
it was constantly present, but that specimens which were starved lost
it in a few days, the more complete the starvation the more thorough
being the loss; it reappeared when they were fed again. Schulze, on the
other hand, believes that it serves, in combination with mucus secreted
by the stomach, to protect the intestine against laceration by sharp
particles introduced with the food. W. Clark found that in _Pholas_ the
stylet is connected with a light yellow corneous plate, and imagined
therefore that it acts as a sort of spring to work the plate in order
to comminute the food, the two together performing somewhat the
function of a gizzard.[330]
5. and 6. _The Intestine, Rectum, and Anus._--The intestine, the wider
anal end of which is called the rectum, almost invariably makes a bend
forward on leaving the stomach. This is the case in the Cephalopoda,
Scaphopoda, and the great majority of Gasteropoda. The exceptions are
the bilaterally symmetrical Amphineura, in which the anus is terminal,
and many Opisthobranchiata, in which it is sometimes lateral (Fig. 68,
p. 159), sometimes dorsal (Fig. 67). The intestine is usually short in
carnivorous genera, but long and more or less convoluted in those which
are phytophagous. In all cases where a branchial or pulmonary cavity
exists, the anus is situated within it, and thus varies its position
according to the position of the breathing organ. Thus in _Helix_ it
is far forward on the right side, in _Testacella_, _Vaginula_, and
_Onchidium_ almost terminal, in _Patella_ at the back of the neck,
slightly to the right side (Fig. 64, p. 157).
In the rhipidoglossate section of the Diotocardia (_Trochus_,
_Haliotis_, etc.) the rectum passes through the ventricle of the heart,
a fact which, taken in conjunction with others, is evidence of their
relationship to the Pelecypoda.
[Illustration: FIG. 147.--Ink-sac of _Sepia_, showing its
relation to the rectum: _a_, anus; _d_, duct of sac; _i.g_,
ink-gland; _i.r_, portion of the sac which serves as a reservoir
for the ink; _o_, orifice of ink-gland; _r_, rectum; _sp_, double
set of sphincter muscles controlling upper end of duct. (Modified
from Girod.)]
In nearly all Pelecypoda the intestine is very long and convoluted,
being sometimes doubled forward over the mouth. Towards its terminal
part it traverses the ventricle of the heart, except in _Ostrea_,
_Anomia_, _Teredo_, and a few more. The anus is always at the posterior
end of the animal, adjacent to and slightly above the adductor muscle.
Anal glands, which open into the rectum close to the anus, are present
in some Prosobranchiata, _e.g._ _Murex_, _Purpura_. In the Cephalopoda
the anal gland becomes of considerable size and importance, and is
generally known as the ink-sac (Fig. 147); it occurs in all known
living genera, except _Nautilus_. The ink-sac consists of a large bag
generally divided into two portions, in one of which the colouring
matter is secreted, while the other acts as a reservoir for its
storage. A long tube connects the bag with the end of the rectum, the
mouth of the tube being controlled, in _Sepia_, by a double set of
sphincter muscles.
=The Kidneys=
The kidneys, nephridia,[331] renal or excretory organs, consist
typically of two symmetrical glands, placed on the dorsal side of
the body in close connexion with the pericardium. Each kidney opens
on the one hand into the mantle cavity, close to the anus (see Fig.
64, p. 157), and on the other, into the pericardium. The venous blood
returning from the body passes through the vascular walls of the
kidneys, which are largely formed of cells containing uric acid. The
blood thus parts with its impurities before it reaches the breathing
organs.
The kidneys are paired in all cases where the branchiae are paired,
and where the heart has two auricles, _i.e._ in the Amphineura, the
Diotocardia (with the exception of the Neritidae), the Pelecypoda,
and all Cephalopoda except _Nautilus_, which has four branchiae,
four auricles, and four kidneys. In other Gasteropoda only one
kidney survives, corresponding to the left kidney of Zygobranchiate
Gasteropods.
Besides their use as excretory organs the kidneys, in certain groups of
the Mollusca, stand in very close relation to the genital glands. In
some of the Amphineura the generative products, instead of possessing
a separate external orifice of their own, pass from the genital gland
into the pericardium and so out through the kidneys (see Fig. 61 C, D,
p. 154). In the Diotocardia it is the right kidney alone which serves,
besides its excretory functions, as a duct for the emission of the
generative products, the left kidney being at the same time greatly
reduced in size. Thus in _Patella_ the left nephridium is small, the
right being much larger; both function as excretory organs, but the
right serves as a mode of conveyance for the seminal products as well.
In certain Pelecypoda (_e.g._ _Yoldia_, _Avicula_, _Modiola_, _Pecten_,
_Spondylus_) the genital glands communicate directly, and with a
similar object, with the renal pouch on the same side of the body, but
in the majority of cases the orifices are distinct.
* * * * *
The following memoirs will be found useful for further study of this
portion of the subject:--
=D. Barfurth=, Ueber den Bau und die Thätigkeit der
Gasteropodenleber: Arch. Mikr. Anat. xxii. (1883), pp. 473–524.
=Th. Behme=, Beiträge zur Anatomie und Entwickelungsgeschichte
des Harnapparates der Lungenschnecken: Arch. Naturges. iv.
(1889), pp. 1–28.
=R. Bergh=, Semper’s Reisen im Archipelago der Philippinen;
Nudibranchiata: Theil ii. Band ii. (1870–78), Band iii.
(1880–1892).
=W. G. Binney=, Terrestrial Air-breathing Mollusks of the United
States: Bull. Mus. C. Z. Harv. iv. (1878), 450 pp.
„ On the Jaw and Lingual Membrane of North American Terrestrial
Pulmonata: Proc. Ac. Nat. Sc. Philad. (1875), pp. 140–243.
=J. T. Cunningham=, The renal organs (Nephridia) of Patella:
Quart. Journ. Micr. Sc. xxiii. (1883), pp. 369–375.
„ „ Note on the structure and relations of the kidney in
Aplysia: Mitth. Zool. Stat. Neap. iv. (1883), pp. 420–428.
=R. von Erlanger=, On the paired Nephridia of Prosobranchs,
etc.: Quart. Journ. Micr. Sc. xxxiii. (1892), pp. 587–623.
=H. Fischer=, Recherches sur la Morphologie du Foie des
Gastéropodes: Bull. Scient. France Belg. xxiv. (1892), pp.
260–346.
=C. Grobben=, Morphologische Studien über den Harn- und
Geschlechtsapparat, sowie die Leibeshöhle, der Cephalopoden:
Arb. Zool. Inst. Wien, v. (1884), pp. 179–252.
„ Die Pericardialdrüse der Gasteropoden: ibid. ix. (1890), pp.
35–56.
=B. Haller=, Beiträge zur Kenntniss der Niere der
Prosobranchier: Morph. Jahrb. xi. (1885), pp. 1–53.
=A. Hancock=, On the structure and homologies of the renal organ
in the Nudibranchiate Mollusca: Trans. Linn. Soc. xxiv. (1864),
pp. 511–530.
=A. Köhler=, Microchemische Untersuchung der Schneckenzungen:
Zeits. Gesamm. Naturw. viii. (1856), pp. 106–112.
=Ad. Oswald=, Der Rüsselapparat der Prosobranchier: Jena. Zeits.
Naturw. N.F. xxi. (1893), pp. 114–162.
=R. Perrier=, Recherches sur l’anatomie et l’histologie du rein
des Gastéropodes prosobranches: Ann. Sc. Nat. Zool. (7), viii.
(1889), pp. 61–315.
=C. Semper=, Reisen im Archipelago der Philippinen; Land
Pulmonata: Theil ii. Band iii. (1870–77).
=C. Troschel=, Das Gebiss der Schnecken: Berlin, 1856–1892.
=W. G. Vigelius=, Ueber das Excretionssystem der Cephalopoden:
Niederl. Arch. Zool. v. (1880), pp. 115–184.
CHAPTER IX
THE SHELL, ITS FORM, COMPOSITION AND GROWTH--DESIGNATION
OF ITS VARIOUS PARTS
The popular names of ‘shells,’ ‘shell-fish,’ and the like, as commonly
applied to the Mollusca, the intrinsic beauty and grace of the
shells themselves, resulting in the passion for their collection,
their durability and ease of preservation, as compared with the
non-testaceous portion,--all these considerations tend to unduly exalt
the value of the shell as part of the organism as a whole, and to
obscure the truth that the shell is by no means the most important of
the organs.
At the same time it must not be forgotten that the old systems of
classification, which were based almost entirely on indications drawn
from the shell alone, have been strangely little disturbed by the new
principles of arrangement, which depend mainly on structural points in
the animal. This fact only tends to emphasise the truth that the shell
and animal are in the closest possible connexion, and that the shell
is a living part of the organism, and is equally sensitive to external
influences.
A striking instance of the comparative valuelessness of the shell alone
as a primary basis of classification is furnished by the large number
of cases in which a _limpet-shaped_ shell is assumed by genera widely
removed from one another in cardinal points of organisation. This
form of shell occurs in the common limpet (Patellidae), in _Ancylus_
(Limnaeidae), _Hemitoma_ (Fissurellidae), _Cocculina_ (close to
Trochidae), _Umbrella_ and _Siphonaria_ (Opisthobranchiata), while in
many other cases the limpet form is nearly approached.
Roughly speaking, about three-quarters of the known Mollusca, recent
and fossil, possess a univalve, and about one-fifth a bivalve shell.
In _Pholas_, and in some species of _Thracia_, there is a small
accessory hinge plate; in the Polyplacophora, or Chitons, the shell
consists of eight plates (see Fig. 2, p. 8), usually overlapping. A
certain proportion of the Mollusca have no shell at all. In many of
these cases the shell has been present in the larva, but is lost in the
adult.
The shell may be
(1) _External_, as in the great majority of both univalves and
bivalves.
(2) _Partly external_, _partly internal_; _e.g._ _Homalonyx_,
_Hemphillia_, some of the _Naticidae_, _Scutum_, _Acera_, _Aplustrum_
(Figs. 148 and 149).
[Illustration: FIG. 148.--_Aplustrum aplustre_ L. Mauritius,
showing the partly internal shell (=S=); =F=, foot; =LL=,
cephalic lappets; =TT=, double set of tentacles. (After Quoy and
Gaimard.)]
[Illustration: FIG. 149.--_Sigaretus laevigatus_ Lam., showing
shell partially immersed in the foot; =F=, anterior prolongation
of the foot. (After Souleyet.)]
(3) _Internal_; _e.g._ _Philine_, _Gastropteron_, _Pleurobranchus_,
_Aplysia_, _Limax_, _Arion_, _Hyalimax_, _Parmacella_, _Lamellaria_,
_Cryptochiton_, and, among bivalves, _Chlamydoconcha_.
(4) _Absent_; _e.g._ all _Nudibranchiata_ and _Aplacophora_, many
_Cephalopoda_, a few land Mollusca, _e.g._ all _Onchidiidae_,
_Philomycus_, and _Vaginula_.
=The Univalve Shell.=--In univalve Mollusca the normal form of the
shell is an elongated cone twisted into a spiral form round an axis,
the spiral ascending to the left. Probably the original form of the
shell was a simple cone, which covered the vital parts like a tent.
As these parts tended to increase in size, their position on the
dorsal side of the animal caused them gradually to fall over, drawing
the shell with them. The result of these two forces combined, the
increasing size of the visceral hump, and its tendency to pull the
shell over with it, probably resulted in the conversion of the conical
into the spiral shell, which gradually came to envelop the whole
animal. Where the visceral hump, instead of increasing in size, became
flattened, the conical shape of the shell may have been modified into a
simple elliptical plate (_e.g._ _Limax_), the nucleus representing the
apex of the cone. In extreme cases even this plate dwindles to a few
calcareous granules, or disappears altogether (_Arion_, _Vaginula_).
=Varieties of the Spiral.=--Almost every conceivable modification of
the spiral occurs, from the type represented by _Gena_, _Haliotis_,
_Sigaretus_, and _Lamellaria_, in which the spire is practically
confined to the few apical whorls, with the body-whorl inordinately
large in proportion, to a multispiral form like _Terebra_, with about
twenty whorls, very gradually increasing in size.
[Illustration: FIG. 150.--Examples of shells with =A=, a
flattened spire (_Polygyratia_); =B=, a globose spire (_Natica_);
=C=, a greatly produced spire (_Terebra_).]
As a rule, the spire is more or less obliquely coiled round the axis,
each whorl being partially covered, and therefore hidden by, its
immediate successor, while the size of the whorls, and therefore the
diameter of the spire as a whole, increases somewhat rapidly. The
effect of this is to produce the elevated spire, the shell of six to
ten whorls, and the wide aperture, of the normal type of mollusc, the
whelk, snail, periwinkle, etc.
Sometimes, however, the coil of the whorls, instead of being oblique,
tends to become horizontal to the axis, and thus we have another series
of gradations of form, from the excessively produced spire of _Terebra_
to the flattened disc of _Planorbis_, _Polygyratia_, _Euomphalus_, and
_Ammonites_. The shell of many species of _Conus_ practically belongs
to the latter type, each whorl folding so closely over its predecessor
that the spiral nature of the shell is not perceived until it is looked
at at right angles to the spire.
[Illustration: FIG. 151.--Examples of shells with disconnected
whorls; =A=, _Cyathopoma cornu_ Mf., Philippines; =B=,
_Cylindrella hystrix_ Wright, Cuba. (Both × 4.)]
[Illustration: FIG. 152.--Example of a shell whose apical whorls
alone are coiled, and the remainder produced in a regular curve.
(_Cyclosurus Mariei_ Morel., Mayotte.)]
In some cases the regularly spiral form is kept, but the whorls are
completely disconnected; _e.g._ some _Scalaria_, _Spirula_; among
fossil Cephalopoda, _Gyroceras_, _Crioceras_, and _Ancyloceras_; and,
among recent land Mollusca, _Cylindrella hystrix_ and _Cyathopoma
cornu_ (Fig. 151). Sometimes only the last whorl becomes disconnected
from the others, as in _Rhiostoma_ (see Fig. 180, p. 266),
_Teinostoma_, and in the fossil _Ophidioceras_ and _Macroscaphites_.
Sometimes, again, not more than one or two whorls at the apex are
spirally coiled, and the rest of the shell is simply produced or
coiled in an exceedingly irregular manner, _e.g._ _Cyclosurus_,
_Lituites_, _Orygoceras_, _Siliquaria_ (Fig. 153), _Vermetus_. In
_Coecum_ (Fig. 170, p. 260) the spiral part is entirely lost, and the
shell becomes simply a cylinder. In a few cases the last whorl is
coiled irregularly backwards, and is brought up to the apex, so that
the animal in crawling must carry the shell with the spire downwards,
as in _Anostoma_ (Fig. 154), _Opisthostoma_ (Fig. 208, p. 309),
_Strophostoma_, and _Hypselostoma_ (Fig. 202 A, p. 302).
[Illustration: FIG. 153.--_Siliquaria anguina_ Lam., showing
scalariform coil of upper whorls and irregular extension of the
lower.]
[Illustration: FIG. 154.--_Anostoma globulosum_ Lam., Brazil.
(After P. Fischer.)]
[Illustration: FIG. 155.--Various forms of the internal plate in
_Capulidae_: =A=, _Calyptraea_ (_Mitrularia_) _equestris_ Lam.,
E. Indies; =B=, _Crucibulum scutellatum_ Gray, Panama; =C=,
_Ergaea plana_ Ad., and Reeve, Japan; =D=, _Galerus chinensis_
L., Britain; =E=, _Crepipatella dilatata_ Lam., Callao; =F=,
_Trochita maculata_ Quoy, N. Zealand; =G=, _Crepidula fornicata_
Lam., N. America.]
Some genera of the _Capulidae_, in which the shell is of a broadly
conical form or with scarcely any spire, develop an internal plate
or process which serves the purpose of keeping the animal within the
shell, and does the work of a strong attachment muscle. In _Mitrularia_
this process takes the form of a raised horse-shoe; in _Crucibulum_ it
is cup-shaped, with the edge free all round; in _Galerus_, _Ergaea_,
_Crepipatella_, and _Trochita_ we get a series of changes, in which
the edge of the cup adheres to the interior of the shell, and then
gradually flattens into a plate. In _Crepidula_ proper this plate
becomes a regular partition, covering a considerable portion of the
interior (Fig. 155 G). _Hipponyx_ secretes a thin calcareous plate on
the ventral surface of the foot, which intervenes like an operculum
between the animal and the substance to which it adheres.
_Sinistral, or Left-handed Shells._--The vast majority of univalve
spiral shells are normally _dextral_, _i.e._ when held spire uppermost,
with the aperture towards the observer, the aperture is to the right
of the axis of the spire. If we imagine such a shell to be a spiral
staircase, as we ascended it we should always have the axis of the
spire to our left.
Sinistral or ‘reversed’ forms are not altogether uncommon, and may be
grouped under four classes:--
(1) Cases in which the _genus is_ normally sinistral; (2) cases in
which the _genus_ is _normally dextral_, but _certain species_ are
_normally sinistral_; (3) cases in which the shell is _indifferently
dextral or sinistral_; (4) cases in which _both genus and species_ are
_normally dextral_, and a sinistral form is an _abnormal monstrosity_.
[Illustration: FIG. 156.--_Fulgur perversum_ L., Florida. × ½.]
[Illustration: FIG. 157.--Illustration of the gradation of forms
in _Ampullaria_ between a dextral (=A=) and an ultra-dextral
species (=F=).]
In all cases of sinistral monstrosity, and all in which a sinistral and
dextral form are interchangeable (sections 3 and 4 above), the position
of the apertures of the internal organs appears to be relatively
affected, _i.e._ the body is sinistral, as well as the shell. This has
been proved to be the case in all specimens hitherto examined, and may
therefore be assumed for the rest. The same uniformity, however, does
not hold good in all cases for genera and species normally sinistral
(sections 1 and 2). As a rule, the anal and genital apertures are, in
these instances also, to the left, but not always. In _Spirialis_,
_Limacina_, _Meladomus_, and _Lanistes_ the shell is sinistral, but
the animal is dextral. This apparent anomaly has been most ingeniously
explained by Simroth, Von Ihering, and Pelseneer. The shell, in all
these cases, is not really sinistral, but _ultra-dextral_. Imagine
the whorls of a dextral species capable of being flattened, as in a
_Planorbis_, and continue the process, still pushing, as it were, the
spire downwards until it occupies the place of the original umbilicus,
becoming turned completely ‘inside out,’ and we have the whole
explanation of these puzzling forms. The animal remains dextral, the
shell has become sinistral. A convincing proof of the truth of this
is furnished by the operculum. It is well known that the twist of the
operculum varies with that of the shell; when the shell is dextral, the
operculum is sinistral, with its nucleus near the columella, and _vice
versâ_. In these ultra-dextral shells, however, where it is simply the
method of the enrolment of the spire that comes in question, and not
the formation of the whorls themselves, the operculum remains sinistral
on the apparently sinistral shell.
The reverse case to this, when the shell is dextral but the orifices
sinistral, is instanced by the two fresh-water genera _Pompholyx_ (from
N. America), and _Choanomphalus_ (L. Baikal). A similar transition in
the enrolment of the whorls may be confidently assumed to have taken
place, and the shells are styled _ultra-sinistral_.
Yet another variation remains, in which the embryonic form is
sinistral, but the adult shell dextral, the former remaining across the
nucleus of the spire. This is the case with _Odostomia_, _Eulimella_,
_Turbonilla_, and _Mathilda_, all belonging to the Prosobranchiata,
with _Actaeon_, _Tornatina_, and _Actaeonina_ among the Opisthobranchs,
and _Melampus_ alone among Pulmonates.
=Monstrosities of the Shell.=--Abnormal growths of the shell constantly
occur, some of them being scarcely noticeable, except by a practised
eye, others of a more serious nature, involving an entire change in
the normal aspect of the creature. _Scalariform_ monstrosities are
occasionally met with, especially in _Helix_ and _Planorbis_, when
the whorls become unnaturally elevated, and sometimes quite disjoined
from one another; _carinated_ monstrosities develop a keel on a whorl
usually smooth; _acuminated_ monstrosities have the spire produced to
an extreme length (Fig. 158); _sinistral_ monstrosities (see above)
have the spire reversed: dwarfs and giants, as in our own race, are
occasionally noticed among a crowd of individuals.
More serious forms of monstrosity are those which occur in individual
cases. Mr. S. P. Woodward once observed[332] a specimen of an adult
_Helix aspersa_ with a second, half-grown individual fixed to its
spire, and partly embedded in the suture of the body whorl. The younger
snail had died during its first hibernation, as was shown by the
epiphragm remaining in the aperture, and its neighbour, not being able
to get free of the incubus, partially enveloped it in the course of
its growth. In the British Museum two _Littorina littorea_ have become
entangled in a somewhat similar way (Fig. 160 B), possibly as a result
of embryonic fusion. Double apertures are not uncommon[333] in the
more produced land-shells, such as _Cylindrella_ and _Clausilia_ (Fig.
160 A). In the Pickering collection was a _Helix hortensis_ which had
crawled into a nutshell when young, and, growing too large to escape,
had to carry about this decidedly extra shell to the end of its days.
A monstrosity of the cornucopia form, in which the whorls are uncoiled
almost throughout, is of exceedingly rare occurrence (Fig. 161).
[Illustration: FIG. 158.--Monstrosities of _Neptunea antiqua_ L.,
and _Buccinum undatum_ L., with a greatly produced spire (from
specimens in the Brit. Mus.).]
[Illustration: Fig. 159.--Monstrosities of _Littorina rudis_ Mat,
The Fleet, Weymouth. (After Sykes.)]
Some decades ago ingenious Frenchmen amused themselves by creating
artificial monstrosities. _H. aspersa_ was taken from its shell, by
carefully breaking it away, and then introduced into another shell of
similar size (_H. nemoralis_, _vermiculata_, or _pisana_). At the end
of several days attachment to the columella took place, and then growth
began, the new shell becoming soldered to the old, and the spiral
part of the animal being protected by a thin calcareous envelope. A
growth of from one to two whorls took place under these conditions.
The individuals so treated were always sordid and lethargic, but they
bred, and naturally produced a normal _aspersa_ offspring.[334] In the
British Museum there is a specimen of one of these artificial unions of
a _Helix_ with the shell of a _Limnaea stagnalis_.
[Illustration: FIG. 160.--Monstrosities with two apertures: =A=,
_Cylindrella agnesiana_ C. B. Ad., Jamaica; =B=, _Littorina
littorea_ (from specimens in the British Museum).]
[Illustration: FIG. 161.--Cornucopia-shaped monstrosity of _Helix
aspersa_, from Ilfracombe. (British Museum.)]
=Composition of the Shell.=--The shell is mainly composed of pure
carbonate of lime, with a very slight proportion of phosphate of lime,
and an organic base allied to chitin, known as _conchiolin_. The
proportion of carbonate of lime is known to vary from about 99 p.c. in
_Strombus_ to about 89 p.c. in _Turritella_. Nearly 1 p.c. of phosphate
of lime has been obtained from the shell of _Helix nemoralis_, and
nearly 2 p.c. from that of _Ostrea virginica_. The conchiolin forms a
sort of membranous framework for the shell; it soon disappears in dead
specimens, leaving the shell much more brittle than it was when alive.
Carbonate of magnesia has also been detected, to the extent of ·12 p.c.
in _Telescopium_ and ·48 p.c. in _Neptunea antiqua_. A trace of silica
has also occasionally been found.
When the shell exhibits a crystalline formation, the carbonate of lime
may take the form either of _calcite_ or _aragonite_. The calcite
crystals are rhombohedral, optically uniaxal, and cleave easily, while
the aragonite cleave badly, belong to the rhombic system, and are
harder and denser, and optically biaxal. Both classes of crystal may
occur in the same shell.
Two main views have been held with regard to the formation and
structure of the shell--(1) that of Bowerbank and Carpenter, that the
shell is an organic formation, growing by interstitial deposit, in the
same manner as the teeth and bones of the higher animals; (2) that
of Réaumur, Eisig, and most modern writers, that the shell is of the
nature of an excretion, deposited like a cuticle on the outside of the
skin, being formed simply of a number of calcareous particles held
together by a kind of ‘animal glue.’ Leydig’s view is that the shell
of the Monotocardia is a secretion of the epithelium, but that in the
Pulmonata it originates within the skin itself, and afterwards becomes
free.[335]
According to Carpenter, when a fragment of any recent shell is
decalcified by being placed in dilute acid, a definite animal basis
remains, often so fine as to be no more than a membranous film, but
sometimes consisting of an aggregation of ‘cells’ with perfectly
definite forms. He accordingly divides all shell structure into
_cellular_ and _membranous_, according to the characteristics of the
animal basis. Cellular structure is comparatively rare; it occurs most
notably in _Pinna_, where the shell is composed of a vast multitude of
tolerably regular hexagonal prisms (Fig. 162 B). Membranous structure
comprises all forms of shell which do not present a cellular tissue.
Carpenter held that the membrane itself was at one time a constituent
part of the mantle of the mollusc, the carbonate of lime being secreted
in minute ‘cells’ on its surface, and afterwards spreading over the
subjacent membrane through the bursting of the cells.
The iridescence of _nacreous_ shells is due to a peculiar lineation
of their surface, which can be readily detected by a lens. According
to Brewster, the iridescence is due to the alternation of layers of
granular carbonate of lime and of a very thin organic membrane, the
layers very slightly undulating. Carpenter, on the other hand, holds
that it depends upon the disposition of a single membranous layer
in folds or plaits, which lie more or less obliquely to the general
surface, so that their edges show as lines. The nacreous type of
shell occurs largely among those Mollusca which, from other details
in their organisation, are known to represent very ancient forms
(_e.g._ _Nucula_, _Avicula_, _Trigonia_, _Nautilus_). It is also the
least permanent, and thus in some strata we find that only casts of
the nacreous shells remain, while those of different constitution are
preserved entire.
_Porcellanous_ shells (of which the great majority of Gasteropoda are
instances) usually consist of three layers, each of which is composed
of a number of adjacent plates, like cards on edge. The inclination of
the plates in the different layers varies, but that of the plates in
the inner and outer layer is frequently the same, thus if the plates
are transverse in the middle stratum, they are longitudinal in the
inner and outer strata, and, if longitudinal in the middle, they are
transverse in the other two. Not uncommonly (Fig. 163 B) other layers
occur. In bivalves the disposition and nature of the layers is much
more varied.
[Illustration: FIG. 162.--=A=, Section of shell of _Unio_:
_a_, periostracal layer; _b_, prismatic layer; _c_, nacreous
layer. =B=, Horizontal section of shell of _Pinna_, showing the
hexagonal prisms.]
In _Unio_ the periostracal or uppermost layer is very thin; beneath
this is a prismatic layer of no great depth, while the whole remainder
of the shell is nacreous (Fig. 162 A). Many bivalves show traces of
tubular structure, while in the Veneridae the formation and character
of the layers approaches closely to that of the Gasteropoda. Further
details may be gathered from Carpenter’s researches.[336]
=Formation of Shell.=[337]--The mantle _margin_ is the principal agent
in the deposition of shell. It is true that if the shell be fractured
at any point, the hole will be repaired, thus showing that every part
of the mantle is furnished with shell-depositing cells, but such new
deposits are devoid of colour and of periostracum, and no observation
seems to have been made with regard to the layers of which they are
composed. As a rule the mantle, except at its margin, only serves to
thicken the innermost layer of shell.
It is probable that the carbonate of lime, of which the shell is mainly
composed, is separated from the blood by the epithelial cells of the
mantle margin, and takes the crystalline or granular form as it hardens
on exposure after deposition. The three layers of a porcellanous shell
are deposited successively, and the extreme edge of the mouth, when
shell is forming, will contain only one layer, the outermost; a little
further in, two layers appear, and further still, three. The pigment
cells which colour the surface are situated at the front edge of the
mantle margin.
[Illustration: FIG. 163.--Sections of shells. =A=, _Conus_:
_a_, outer layer; _b_, middle prismatic layer, with obliquely
intersecting laminae above and below; _c_, inner layer. =B=,
_Oliva_: _a_, outer layer; _b_, layer of crossed and curved
laminae; _c_, prismatic layer, succeeded by layer of laminae at
right angles to one another; _d_, inner layer. =C=, _Cypraea_:
_a_, outer layer; _b_, middle layer; _c_, inner layer.]
Shelly matter is deposited, and probably secreted, not only by the
mantle, but also in some genera by the foot. This is certainly the case
in _Cymbium_, _Oliva_, _Ancillaria_, _Cassis_, _Distortio_, and others,
in several of which the foot is so large that the shell appears to be
quite immersed in it.[338]
The deposition of shell is not continuous. Rest periods occur, during
which the function is dormant; these periods are marked off on the
edge of the shell, and are known as _lines of growth_. In some cases
(_Murex_, _Triton_, _Ranella_), the rest period is marked by a decisive
thickening of the lip, which persists on the surface of the shell as
what is called a _varix_ (see p. 263).
[Illustration: FIG. 164.--_Murex tenuispina_ L., Ceylon. × ⅔.]
[Illustration: FIG. 165.--_Neritina longispina_ Récl., Mauritius.
(Operculum removed.)]
The various details of sculpture on the exterior surface of the shell,
the striae, ribs, nodules, imbrications, spines, and other forms
of ornamentation are all the product of similar and corresponding
irregularities in the mantle margin, and have all been originally
situated at the edge of the lip. Spines, _e.g._ those of _Murex_ and
_Pteroceras_, are first formed as a hollow thorn, cleft down its lower
side, and are afterwards filled in with solid matter as the mantle
edge withdraws. What purpose is served by the extreme elaboration
of these spiny processes in some cases, can hardly be considered as
satisfactorily ascertained. Possibly they are a form of sculptural
development which is, in the main, protective, and secures to its
owners immunity from the attacks of predatory fishes.
‘Attached’ genera (e.g. _Chama_, _Spondylus_) when living on smooth
surfaces have a flat shell, but when affixed to coral and other uneven
surfaces they become very irregular in shape. The sculpture of the base
on which they rest is often reproduced in these ‘attached’ shells,
not only on the lower, but also on the upper valve, the growing edge
of which rests on the uneven surface of the base. Oysters attached to
the branches of the mangrove frequently display a central convex rib,
modelled on the shape of the branch, from which the plaits of sculpture
radiate, while specimens fixed to the smooth trunk have no such rib.
_Crepidula_, a genus which is in the habit of attaching itself to other
shells, varies in sculpture according to that of its host. Sometimes
the fact may be detected that a specimen has lived on a ribbed shell
when young, and on a smooth one when old, or _vice versâ_. A new genus
was actually founded by Brown for a _Capulus_ which had acquired ribs
through adhesion to a _Pecten_. A specimen of _Hinnites giganteus_ in
the British Museum must at one period of its growth have adhered to
a surface on which was a Serpula, the impression of which is plainly
reproduced on the upper valve of the _Hinnites_.[339]
[Illustration: FIG. 166.--A specimen of _Anomia ephippium_ L.,
Weymouth, taken upon _Pecten maximus_, the sculpture of which
is reproduced on the upper valve of the _Anomia_, and even on a
young _Anomia_ attached to the larger specimen.]
=Growth of the Shell.=--Nothing very definite is known with regard to
the rate of growth of the shell in marine Mollusca. Under favourable
conditions, however, certain species are known to increase very
rapidly, especially if the food supply be abundant, and if there
is no inconvenient crowding of individuals. Petit de la Saussaye
mentions[340] the case of a ship which sailed from Marseilles for
the west coast of Africa, after being fitted with an entirely new
bottom. On arriving at its destination, the vessel spent 68 days in
the Gambia River, and took 86 days on its homeward voyage. On being
cleaned immediately on its return to Marseilles, an _Avicula_ 78 mm.
and an _Ostrea_ 95 mm. long (both being species peculiar to W. Africa)
were taken from its keel. These specimens had therefore attained this
growth in at most 154 days, for at the period of their first attachment
they are known to be exceedingly minute. P. Fischer relates[341] that
in 1862 a buoy, newly cleaned and painted, was placed in the basin
at Arcachon. In less than a year after, it was found to be covered
with thousands of very large _Mytilus edulis_, 100 mm. × 48 mm., the
ordinary size on the adjoining banks being only about 50 to 60 × 30 mm.
Some observations have already been recorded (p. 40) on the growth of
_Helix aspersa_. In the summer of 1858, which was very dry, especially
in the south of France, the young Helices born that year were still
very small in August. About the end of that month abundant rain came
on, and in four or five days young _H. variabilis_, _H. pisana_, and
_H. aspersa_, eating without cessation, as if to make up for lost
time, grew more than a centimetre of shell. The lip of a young _H.
arbustorum_ has been observed to have grown, at the end of the first
week in the season’s growth, 3 mm., at the end of the second week, 6·25
mm., the third, 11·5 mm., and the fourth 12·5 mm., with a finished
lip.[342]
Careful observation has shown that in the growth of the shell of
_Helix aspersa_ the periostracum is first produced; it is covered with
hyaline globules, 10–12 mm. in diameter, which persist even in the
oldest shells. Calcareous matter is deposited on the internal face of
the new periostracum, at some distance from the margin. It is secreted
by a white zone or band of cells bounding the entire breadth of the
mantle as applied to the peristome. Immediately behind the white zone
are a series of pigment cells which not only give the shell its colour
but complete the calcification of the shelly matter laid down by the
white zone. When the animal has attained its full growth and the lip
is finished off, the white band and the periostracum cells completely
disappear, and only such cells persist as contribute to the internal
thickening of the shell. Shell growth, in this species, is very rapid.
If a portion of the pulmonary sac is laid bare, by removing a fragment
of shell, at the end of 1½ or 2 hours there may be detected a delicate
organic membrane covering the hole, and strewn with crystals of
carbonate of lime. This thickens with great rapidity, and soon fills
up the hole with solid matter. For two consecutive months an animal,
deprived of food, has been known to reproduce this membrane daily after
its removal every morning.[343] Prof. Schiedt has found that oysters,
if deprived of the right valve and exposed to the light, not only
develop brown pigment over the whole exposed surface of mantle and
branchiae, but actually succeed in part in reproducing the valve and
hinge.[344]
_Deposit of Additional Layers of Shell._--Mollusca possess the power
of thickening the interior of the shell, by the deposit of successive
layers. This is frequently done in self-defence against the attacks
of boring Mollusca, sponges, and worms. Cases may often be noticed of
_Ostrea_, _Spondylus_, and other sedentary molluscs, which, unable
to escape the gradual assaults of their foes, have provided against
them by the deposit of fresh shelly matter. A somewhat similar plan is
adopted to provide against intrusion by way of the aperture. Pearls
are, in many cases, the result of shell deposition upon the eggs or
even the body of some intrusive parasite (_Distoma_, _Filaria_, etc.),
and are, in some countries, artificially produced by the introduction
of fragments of sand, metal, etc., into living _Unio_ and _Anodonta_.
Little joss images are made in India and China, the nacre on which is
produced by thrusting them inside living Unionidae.
A specimen of _Helix rosacea_, in the British Museum, into whose shell
a piece of grass somehow became introduced, has partitioned it off
by the formation of a sort of shelly tunnel extending throughout its
entire length (Fig. 167).
[Illustration: FIG. 167.--A specimen of _Helix rosacea_ Müll.,
Cape of Good Hope, into which a piece of grass has by some means
become introduced. The animal has protected itself by covering
the grass with a shelly layer. (From a specimen in the British
Museum.)]
=Absorption of Internal Portions.=--Certain genera have the remarkable
property of absorbing, when they become adult, the internal portions
of the whorls and the greater part of the columellar axis. The
effect of this is to make the shell, when the process is complete,
no longer a spiral but a more or less produced cone, and it is found
that in such cases the viscera of the spire lose their spiral form,
and take the shape of the cavity in which they lie. Amongst the
genera in which this singular process takes place are _Nerita_,[345]
_Olivella_, and _Cypraea_ amongst marine forms, and nearly the whole
of the Auriculidae[346] (Fig. 168). _Conus_ reduces the internal
subdivisions of the spire to extreme thinness. It is noticeable that
these genera are all of considerable thickness of shell, and it is
perhaps the result of the whole energy of the animal being directed to
the formation of its external protection that the internal walls of the
spire become atrophied and eventually disappear.
[Illustration: FIG. 168.--_Auricula Judae_ Lam., showing the
disappearance of the partitions of the whorls, which are
represented by dotted lines. (After Fischer.)]
[Illustration: FIG. 169.--=A=, Decollated (adult) form, and =B=,
perfect (young) form of _Cylindrella nobilior_ Ad., Jamaica; the
dotted line shows where decollation takes place.]
[Illustration: FIG. 170.--Development of _Coecum_: =A=, showing
the gradual formation of septa; _a_, apex; _ap_, aperture; _ss_,
first septum; _s´s´_, second septum. (After de Folin.) =B=, Adult
form of _C. eburneum_ Ad., Panama. × 8.]
=Decollation.=--In certain genera, when the shell becomes adult, the
animal ceases to occupy the upper whorls, which accordingly die and
drop off, the orifice at the top having meanwhile been closed by a
shelly deposit. Such shells are termed _decollated_. In some land
genera decollation is the rule, _e.g._ in _Cylindrella_ (Fig. 169),
_Eucalodium_, and _Rumina_, as well as in many species of the brackish
water genera, _Truncatella_, _Cerithidea_, and _Quoyia_. _Stenogyra_
(_Rumina_) _decollata_, a common shell in the south of Europe, has
been noticed to bang its upper whorls violently against some hard
substance, as if to get rid of them.
[Illustration: FIG. 171.--Four stages in the growth of
_Fissurella_, showing how the spire gradually disappears and
the marginal slit becomes an apical hole, =A=, =B=, =C=, highly
magnified, =D=, natural size. (After Boutan.)]
[Illustration: FIG. 172.--Three stages in the growth of _Cypraea
exanthema_ L. (From specimens taken at Panama.)]
=Special Points in the Growth of Certain Genera.=--In the young of
_Coecum_ the apex is at first spiral, but as growth proceeds and the
long tube begins to form, a septum is produced at the base of the apex,
which soon drops off. Soon afterwards, a second septum forms a little
farther down, and a second piece drops off, leaving the shell in the
normal cylindrical form of the adult (Fig. 170). The development of
_Fissurella_ is of extreme interest. In an early stage it possesses
a spiral shell, with a slit on the margin of the outer lip of the
last whorl. As growth advances, shelly matter is deposited on both
margins, which results in the slit becoming a hole and the spire a
mere callosity, until at last they appear to coalesce in the apex of
the adult shell (Fig. 171). The singular formations of _Magilus_ and
_Rhizochilus_ have already been described (pp. 75, 76). _Cypraea_, in
the young stage, is a thin spiral shell with a conspicuous apex. As
growth proceeds, the surface of the whorls, which are nearly enveloped
by two large lobes of the mantle, becomes overlaid with new layers
of shelly matter, until eventually the spire becomes embedded, and
ultimately disappears from view (Fig. 172).
_Patella_, when young, has a nautiloid shell (see Fig. 45, p. 134),
but it is a remarkable fact that we are entirely ignorant, in this
commonest of molluscs, of the transition stages which convert the
nautiloid into the familiar conical shell. The young shell of
_Pteroceras_ is deceptively unlike the adult, and is entirely devoid
of the finger-like processes which are so characteristic of the genus
(chap. xiv.).
Among the bivalve Mollusca, _Anomia_ in a young stage is not to be
distinguished from Ostrea. Soon a small sinus appears on the ventral
margin, which gradually deepens and, as the shell grows round it, forms
a hole for the byssus, eventually becoming fixed beneath the umbones
(see Fig. 173). In _Teredo_ the two valves of the shell proper, which
is very small, become lodged in a long calcareous tube or cylinder,
which is generally open at both ends (see chap. xvi.). In _Aspergillum_
a somewhat similar cylinder is developed, but the valves are soldered
to the tube, and form a part of it, the tube itself being furnished, at
the anterior end, with a disc, which is perforated with holes like the
rose of a watering-pot. In _Clavagella_ the left valve alone becomes
soldered to the tube, while the right valve is free within it (see
chap. xvi.). _Fistulana_ encloses the whole of its shell in a long
tapering tube, which is not at any point adherent to the shell.
[Illustration: FIG. 173.--Development of the byssus or the
plug-hole in _Anomia_. (After Morse.)]
=Terms employed to denote various Parts of the Univalve Shell.=--The
_apex_ is the extreme top of the spire, and generally consists of
the embryonic shell, which may often be recognised by its entire
want of sculpture. When the embryonic shell happens to be large, the
apex is often mammillated, _e.g._ in _Fusus_, _Neptunea_, and some
_Turbinella_; in the _Pyramidellidae_ it is sinistral.
The _suture_ is the line of junction between any two successive
whorls. It may be deep, and even channelled, or very shallow, as in
Fig. 150 B (p. 246).
The _spire_ is the whole series of whorls except the last or _body
whorl_. A _whorl_ is a single revolution of the spiral cone round
the axis. The spire may be _subulate_ (as in _Terebra_, Fig. 150 C),
_turreted_ (_Scalaria_), _depressed_ (_Polygyratia_, Fig. 150 A),
_conical_ (_Trochus_), _globose_ (_Ampullaria_, _Natica_, Fig. 150
B), with almost all conceivable gradations between these types. The
number of whorls is best counted by placing the shell mouth downwards,
and reckoning _one_ for every suture that occurs between the extreme
anterior point of the shell and the apex.
[Illustration: FIG. 174.--Illustrating the technical terms
applied to the various parts of a univalve shell.]
The _mouth_ or aperture may be (_a_) entire, as in _Helix_, _Natica_,
_Ampullaria_, when its _peristome_ or margin is not interrupted by any
notch or canal, or (_b_) prolonged at its anterior and sometimes also
at its posterior end into a _canal_. The _anterior canal_ serves as a
protection to the siphon,[347] the _posterior canal_ is mainly anal
in function, and corresponds, in part, to the hole of _Fissurella_,
the slit in _Pleurotoma_ and _Emarginula_, and the row of holes in
_Haliotis_. The mouth presents every variety of shape, from the perfect
circle in _Cyclostoma_ and _Trochus_, to the narrow and prolonged slit
in _Conus_ and _Oliva_.
[Illustration: FIG. 175.--Anal slit in _Pleurotoma_.]
The right margin of the mouth (the left, in sinistral shells) is
termed the _outer lip_ or _labrum_, the left margin the _inner lip_,
_labium_, or _columella lip_.[348] In young shells the outer lip
is usually thin and unfinished, while in the adult it is generally
thickened into a rib, or furnished with more or less prominent teeth,
or given an inward or outward curve. In some genera, especially the
Strombidae, the outer lip of the adult develops long finger-like
processes, which sometimes attain an extraordinary size (chap.
xiv.). As growth proceeds, these marginal teeth and ribs are either
dissolved and disappear, or are permanently incorporated, in the shape
of _varices_, with the framework of the shell. Some shells, _e.g._
_Natica_, _Turritella_, _Actaeon_, have a permanently unfinished
outer lip, even in the adult stage. The columella lip varies in shape
with the mouth as a whole; thus it may be straight, as in _Conus_, or
excavated, as in _Sigaretus_, _Struthiolaria_, and _Bulla_. Frequently
it is continued by part of the body whorl, as in _Ficula_, _Dolium_,
and _Fasciolaria_.
[Illustration: FIG. 176.--_Solarium perspectivum_ Lam., from the
under side.]
[Illustration: FIG. 177.--Section of _Turbinella pyrum_ L.,
showing the plicae on the columella and the growth of successive
whorls.]
The folds or plaits on the columella, which are often characteristic
of the genus or even family (_e.g._ Fasciolariidae, Mitridae,
Turbinellidae) are not merely external, but continue down the whole
spire (see Fig. 177, which also shows how successive fresh growths have
thickened the columella).
The whorls may be wound in a spiral, which is either hollow, as in
_Solarium_, or quite compact, as in _Oliva_, _Terebra_, _Cypraea_,
with every possible intermediate grade. This concavity, which
varies in depth and width, is known as the _umbilicus_, and shells
are accordingly spoken of as _deeply_ (_e.g._ most Trochidae and
Naticidae), _narrowly_ (_e.g._ _Lacuna_, _Littorina_), or _widely_
(_e.g._ _Solarium_) _umbilicated_. When the spiral is quite flat, as in
_Planorbis_ and some _Helix_, the umbilicus vanishes entirely. Shells
in which the whorls are so compactly coiled on an ascending spiral that
there is no umbilicus, are termed _imperforate_.
[Illustration: FIG. 178.--The slit in =A=, _Hemitoma_, =B=,
_Emarginula_, =C=, _Macroschisma_, =D=, _Craniopsis_, =E=,
_Puncturella_, =F=, _Fissurella_.]
_The Slit._--Many shells are furnished with a slit in the last whorl,
which opens, in most cases, on the outer lip, and is sometimes of
considerable depth, at others a mere notch. In the patelliform shells
it is always in front of the apex. The function of the slit appears to
be mainly anal, the excretory products being thus allowed to escape
by a passage of their own, without soiling the clean water taken in
by the branchiae. The posterior canal of some Gasteropoda probably
performs a similar function. In the adult _Fissurella_ the slit
becomes an apical hole (see Fig. 178 F), in the allied genera it is
either immediately in front of the spire (_Puncturella_), or half-way
between the spire and the anterior margin (_Rimula_), or on the margin
and well marked (_Emarginula_), or a mere indentation of the margin
(_Hemitoma_). In _Pleurotomaria_ it is exceptionally long, and is well
marked in _Bellerophon_, _Schismope_, _Scissurella_, _Murchisonia_, and
_Pleurotoma_ (where it is sutural). In _Haliotis_ and _Polytremaria_
it is replaced by a series of holes, which are closed up as the
animal grows past them. Some of these holes (at least in _Haliotis_)
certainly serve the purpose of admitting water to the branchiae, while
others are anal. In _Trochotoma_ there are only two holes, united by a
narrow fissure.
_The Tubed Land Operculates._--A group of the Cyclophoridae, which is
restricted to Further India and the great Malay Islands, has developed
a remarkable _sutural tube_ on the exterior of the last whorl, near
the aperture, A similar tube, but more obscure, exists in _Alycaeus_.
Several stages in the development of this tube may be noticed,
beginning with the elevation of part of the peristome into a simple
irregular shelly plate, which is continued, first into a short, and
then into a long tube, which becomes soldered to the shell; finally,
the tube becomes free, and the anterior part of the last whorl is
disconnected from the spire (Fig. 180 A-D).
[Illustration: FIG. 179.--The slit in =A=, _Bellerophon_, =B=,
_Pleurotomaria_, =C=, _Schismope_, =D=, _Polytremaria_, =E=,
_Haliotis_ (not drawn to scale).]
[Illustration: FIG. 180.--Development of the tube in the
tube operculates: =A=, _Pterocycus rupestris_ Bens.; =B=,
_Opisthoporus birostris_ Pfr.; =C=, _Spiraculum travancoricum_
Bedd.; =D=, _Rhiostoma Housei_ Pfr.]
It is singular that the tube does not appear to be of any use to
the animal, since its internal extremity, in the complete form, is
closed, and does not communicate with the interior of the whorl. It
may be presumed, however, that in origin the tube served as a means of
conveying air to the animal when the operculum was closed. The holes in
the peristome of _Pupina_, _Cataulus_, and _Anostoma_ (Fig. 154) may be
compared.
[Illustration: FIG. 181.--_Eburna spirata_ Lam., E. Indies. =F=,
foot; =OP=, operculum; =P=, penis; =S=, siphon; =T=, tentacles,
with eyes at their base. (After Souleyet.)]
=The Operculum.=--The operculum is a cuticular development of a
group of cells situated on the dorsal side of the foot, exactly over
the terminal point of the fibres of the columellar muscle. It is so
situated that in crawling it is generally carried free of the shell,
sometimes at the extreme upper end of the foot, more usually somewhat
nearer to the shell (Fig. 181). In _Pterocyclus_ it is pushed back into
the umbilicus when the animal is in motion.
The operculum is present in nearly all land, fresh-water, and marine
Prosobranchiata, absent in all Opisthobranchiata in the adult state,
except _Actaeon_, and in all Pulmonata, except _Amphibola_. It has
been lost in the following marine Prosobranchiata: many Cancellariidae
and Conidae, _Oliva_ (though present in _Olivella_ and _Ancilla_),
Harpidae, Marginellidae, _Voluta_ proper (though present in _V.
musica_), nearly all Mitridae, Cypraeidae, Doliidae, Ianthinidae; and,
of land genera, in Proserpinidae. It is evident, therefore, that its
presence or absence is of limited value in classification. In some
species of _Ampullaria_ and _Natica_ it is horny, in others shelly.
Dall found that in a number of specimens of _Volutharpa ampullacea_,
15 p.c. had opercula, 10 p.c. traces of the operculigenous area, but
no operculum, the rest no trace of either. Monstrosities of _Buccinum
undatum_ sometimes occur, which have two, or in rare case three
opercula.
As a rule, the operculum exactly fits the mouth of the shell. But
in cases where the mouth is very large (_e.g._ _Conus_, _Strombus_,
_Concholepas_, some _Bullia_), it only covers a very small portion and
is quite inadequate as a protection (Fig. 62, p. 155). Again, when the
shell has assumed a more or less limpet-shaped form, and habitually
adheres to flat surfaces without much occasion for locomotion, the
operculum becomes degraded and is probably on the way to being lost
altogether. This is the case with _Navicella_ (a modified _Nerita_, see
Fig. 13, p. 17), _Concholepas_ (a modified _Purpura_), _Sigaretus_ (a
modified _Natica_). Probably the more completely patelliform shells of
_Crepidula_, _Haliotis_, _Fissurella_, and _Patella_ have reached the
stage at which the operculum has been lost entirely. In _Navicella_,
besides becoming degraded, the operculum has actually become partly
internal, and apparently serves the purpose of separating the viscera
from the upper part of the foot, something like the shelly plate in
_Crepidula_. This explains why the operculum in this genus is polished
on both sides.[349]
Some authors have imagined that the operculum is homologous (_a_)
to the second valve in Pelecypoda, (_b_) to the byssus. It differs,
however, morphologically from the former in the essential point of not
being produced by the mantle, and from the latter in not being produced
by a special gland.
[Illustration:
Turbo Turbo Livona Ampullaria Natica
Sarmaticus) (Callopoma)
FIG. 182.--Various forms of opercula.]
As regards shape and formation, the operculum has usually a more or
less well-marked nucleus which may be central (_e.g._ _Livona_),
sub-central (_Ampullaria_), lateral (_Purpura_), or terminal
(_Pyrula_). As a rule, both the inner and outer surfaces are fairly
flat, but in _Torinia_, _Cyathopoma_, and _Pterocyclus_ the outer
surface is elevated and conically spiral, in some _Turbo_ (_e.g._
_Sarmaticus_) it is covered with raised tubercles resembling coral,
while in others (_e.g._ _Callopoma_) it is scored with a deep trench.
_Aulopoma_, a land genus peculiar to Ceylon, has a paucispiral
operculum with hollow whorls, deceptively like a _Planorbis_; it fits
over the aperture instead of into it. In _Livona_ and most Trochidae
the operculum is cartilaginous and multispiral. In _Strombus_ it is
narrow, curved, and often serrated like a leaf on one of the edges; in
_Conus_ it is narrowly oblong and rather featureless; in _Littorina_,
paucispiral and always cartilaginous. In many cases (_e.g._
_Paludina_) there is no true spiral form, but the striae are concentric
to a nearly central nucleus, and thus give the appearance of a spiral.
The evolution of the operculum in _Navicella_ from _Nerita_ has already
been illustrated (p. 10). _Neritopsis_ has a very remarkable operculum,
the striated appendage of which locks behind the columella of the
shell, like the tooth in the opercula of the Neritidae.
[Illustration:
Pyrula Purpura Littorina Aulopoma Torinia
× 3/2 × 3
Neritopsis Strombus Conus
× 2
FIG. 183.--Various forms of opercula.]
=Terms employed to denote various parts of the Bivalve Shell.=--The
_umbo_, or _beak_, is the apex of the hollow cone, of which each valve
may be regarded as consisting. This apex is usually more or less
twisted: it is markedly spiral in _Isocardia_, _Diceras_, some _Chama_,
and especially _Requienia_, while in _Pecten_, _Lepton_, and others the
spiral is altogether absent. As a rule the umbones point _forward_,
_i.e._ towards the anterior end of the shell. In _Donax_, _Nucula_, and
_Trigonia_, however, they point backward. The umbones are generally
more or less approximated, but in _Arca_ they are widely separated.
An _equilateral_ shell is one in which the umbones are more or less
central with regard to its anterior and posterior portion, while in
an _inequilateral_ shell the umbones are much nearer one end than the
other. On the other hand, _equivalve_ and _inequivalve_ are terms used
to express the relation of the two valves to one another as a whole.
Thus nearly all bivalve shells are more or less inequilateral, but a
comparatively small proportion are inequivalve.
The _dorsal margin_ is adjacent to, the _ventral margin_ opposite to,
the umbones. The _anterior_ and _posterior margins_ are respectively
the front and hinder edges of the shell.
The muscles which serve to close the valves leave _impressions_ on the
inner surface of each valve. These, when both muscles are present, are
known as the _anterior_ and _posterior adductor impressions_. The
impression produced by the muscular edge of the mantle, which curves
downwards and backwards from the anterior adductor impression, is known
as the _pallial line_. In shells with only one muscle it is represented
by an irregular row of small marks, or disappears altogether
(_Ostrea_). The _pallial sinus_ is produced by the muscles which
retract the siphons, and is most marked in those genera in which the
muscles are powerful and the siphons large (_e.g._ _Tellina_, _Mya_).
It is entirely absent in genera possessing no retractile siphons.
[Illustration: FIG. 184.--Left valve of _Venus gnidia_ L.: =A=,
anterior, =B=, posterior, =C=, dorsal, =D=, ventral margin, =AB=,
length, =CD=, breadth of shell.
_a.m_, anterior; _p.m_, posterior adductor muscle; _p_, pallial
line; _p.s_, pallial sinus; _l_, ligament; _lu_, lunule; _u_,
umbo; _c_, cardinal teeth; _a.l_, anterior lateral tooth; _p.l_,
posterior lateral tooth.]
[Illustration: FIG. 185.--Right valve of _Lucina tigerina_ L.:
=A=, anterior, =B=, posterior, =C=, dorsal, =D=, ventral margin;
=AB=, length, =CD=, breadth of shell.
_a.m_, anterior; _p.m_, posterior adductor muscles; _p_, pallial
line; _l_, ligament; _u_, umbo; _c_, cardinal teeth; _a.l_,
_p.l_, anterior and posterior lateral tooth.]
_Right and Left Valve._--The simplest way of distinguishing the
valves as right and left is to hold the shell in such a way that the
siphons point towards the observer, and the mouth away from him; in
this position the valve to the right is called the _right valve_, and
the valve to the left the _left valve_. If, however, the animal is
not present, it may be remembered that the ligament is nearly always
_behind_ the beaks, and that the beaks, as a rule, point _forward_,
thus the right and left valves can generally be named by observation
of the beaks and ligament. When the ligament is median to the valves
(_e.g._ _Ostrea_, _Pecten_), and the beaks are not curved, the
valves may be recognised by noting the fact that the impression of
the adductor muscle (in these cases always single) is nearer to the
posterior than to the anterior side. In a similar way the pallial
impression, which only forms a sinus on the posterior side, furnishes a
guide to the valves of _Donax_, in which the beaks point backward, and
of _Tellina_, in which the beaks are frequently central.
In the fixed inequivalves (_e.g._ _Chama_) it is sometimes the right,
sometimes the left valve which is undermost, but the fixed valve,
whether right or left, is always deep, and the free valve flat.
_Ostrea_ and _Anomia_ are always fixed by the _left_ valve.
The _lunule_ is a well-marked area _in front of_ and close to the
umbones, usually more or less heart-shaped, and limited by a ridge.
Generally it is shallow, but sometimes, as in _Dosinia_, _Opis_, and
some _Cardium_, deeply impressed. A corresponding area _behind_ the
umbones, enclosing the ligament, is called the _escutcheon_ (Fig. 186),
but it seldom occurs.
The _ligament_ is a more or less elastic band, which unites the two
valves along a line adjacent to the umbones. As a rule, the greater
part of the ligament is external to the shell, but it may be entirely
internal. It is placed, normally, _behind_ the umbones, but in a few
cases, when the hinge line is very long (_Arca_, _Pectunculus_),
it extends in front of them as well. The edges of the valves, when
the ligament is mainly external, are more or less excavated for its
reception. When internal it is generally contained in a groove or
spoon-shaped pit, known as the _fossette_ (compare Fig. 187).
[Illustration: FIG. 186.--_Venus subrostrata_ Lam.: _es_,
escutcheon; _li_, ligament; _lu_, lunule; _u_, _u_, umbones.]
The ligament consists of two distinct parts, which may occur together
or separately, the external, or _ligament proper_, and the internal, or
_cartilage_. Only the external portion can be seen when the valves are
closed. As a rule, the two portions are intimately connected with one
another, the ligament folding over the cartilage, but in some cases,
_e.g._ _Mya_, _Mactra_, where the cartilage is lodged within the hinge,
they are completely disconnected (Fig. 187).
In _Pecten_ the external ligament is very thin, and runs along the
dorsal margin, while the internal ligament is large, solid, and
situated in a shallow pit. In _Perna_, where the hinge is toothless,
the ligament is folded into a number of transverse ridges, which fit
into corresponding grooves in the shell.
The ligament proper is _inelastic_ and insoluble in caustic potash.
The cartilage is very elastic, composed of parallel fibres, slightly
iridescent, and soluble in caustic potash.
The operation of the ligament--using the word as including the whole
ligamental process--is in opposition to that of the adductor muscles.
When the latter close the valves, they compress the ligament, an action
which its elasticity resists: thus its operation tends in part towards
keeping the valves open. But when ligament and cartilage are both fully
developed, they work in opposition to one another, the ligament, by its
resistance to compression, preventing any straining of the adductor
muscles when the valves are open, and the cartilage, for the same
reason, preventing the ventral margins of the shell from closing too
rapidly upon one another when the valves are being shut.
[Illustration: FIG. 187.--Hinge of =A=, right valve, and =B=,
left valve of _Mulinia edulis_ King; _ca_, cardinals; _l.a_,
anterior laterals; _l.p_, posterior laterals; _f_, fossette; _c_,
cartilage; _l_, ligament.]
_The Hinge._--The valves of _Pelecypoda_ are generally articulated,
below the umbones, by a _hinge_ which is furnished, in the majority of
cases, with interlocking teeth, small pits or depressions in each valve
corresponding to the teeth in the other. The teeth are distinguished
as _cardinal_, or those immediately below the umbo, and _lateral_,
or those to either side of the cardinals, the latter being also
distinguished as _anterior_ and _posterior laterals_, according as they
are before or behind the umbo (Fig. 184). In shells which are tolerably
equilateral there is no difficulty in distinguishing between cardinal
and lateral teeth. But when they are very inequilateral, the whole
hinge may share in the inequality of growth, and an anterior lateral
may be thrown backward and simulate a cardinal, or a cardinal may be
thrown backward and simulate a posterior lateral (_e.g._ _Cardita_,
_Unio_, Fig. 188). In many _Chama_ the cardinals are pushed up into
the umbo and become a mere ridge, while the strong anterior lateral
becomes nearly central and simulates a cardinal.
[Illustration: FIG. 188.--Hinges of =A=, _Cardita semiorbiculata_
Brug., and =B=, _Unio rectus_ Lam., showing how, in inequilateral
shells, the lateral teeth tend to shift their position. _a.m_,
anterior adductor, _p.m_, posterior adductor muscle; _c_, _c_,
cardinal teeth; _p.l_, posterior lateral teeth; _l_, ligament.]
Some bivalves, _e.g._ _Anodonta_, _Ostrea_, _Pedum_, many _Mytilus_,
have no hinge teeth at all, in others the laterals are wanting
(_Psammobia_, _Diplodonta_). In the Arcadae the hinge consists of a
number of very similar denticles, which are often serrated like the
teeth of a comb (Fig. 189).
[Illustration: FIG. 189.--The hinge in _Arcadae_: =A=, _Nacula
Loringi_ Ang. × 4/3; =B=, _Arca granosa_ L.; _u.a_, umbonal area.]
[Illustration: FIG. 190.--=A=, _Tridacna scapha_ Lam.; =B=,
_Cardium enode_ Sowb., showing the interlocking of the ventral
margins.]
Hinge-teeth are probably, in origin, derived from the crenulations or
ribbings of the surface of the shell, the upper ends of which impinge
upon the dorsal margin and mark it in a way which is quite recognisable
when the shell is thin. Similar crenulations, resulting in interlocking
of the valves, are not uncommon upon the ventral margin in certain
genera (Fig. 190). The mechanical effect of these continued riblets,
when fitted together on the opposing valves, would be to prevent the
valves sliding upon one another while closing, or after being closed.
Thus there would be a probability of their surviving, even after the
ribbing had disappeared from the surface of the shell, the increased
strength given by the hinge compensating for, and making it possible to
do without, the extra strength supplied by the ribs. It is therefore
possible that the teeth of the Nuculidae and Arcadae, which have no
distinction between cardinals and laterals, represent a very ancient
type, from which have been evolved the various forms of hinge in which
cardinals and laterals are distinguished. Even in some forms of Arcadae
(comp. _Pectunculus_) we get a hint how the transverse teeth of the
typical _Arca_ may have become transformed into the longitudinal tooth
of the normal lateral.[350]
The developed hinge-teeth, then, ensure the opening of the valves in
one direction; they also secure their accurate closure upon one another
in exactly the same plane. Exposed shells and gaping siphons matter
little to animals which are protected by their burrowing propensities,
but to those which live in material which can be easily penetrated
by their foes, it must be of advantage to be able to buckle their
armour absolutely tight. The edentulous hinge of _Anodonta_
is a degeneration from a dentate type, which retains its teeth (in
_Unio_, etc.) when subject to the jar of rapid streams, but tends
to lose them in the stiller waters of canals, lakes, and ponds.
_Other processes in the bivalve shell._--In _Anatina_ each umbo is
fissured and strengthened on the inside by a kind of umbonal plate
which carries the ligament. Some forms of _Liligna_ develop a strong
internal umbonal rib, which serves as a buttress to strengthen the
shell. In _Pholas_, the so-called falciform process serves as a point
of attachment for the muscles of the foot and viscera. There is no
ligament or hinge-teeth, the place of the latter being taken by the
anterior adductor muscle, which is attached to the hinge-plate, the
latter being reflected back into the shell.
In _Septifer_ the anterior adductor muscle is carried on a sort of
shelf or _myophore_, and in _Cucullaea_ the posterior adductor is
partly raised on a similar and very prominent formation.
_Length and breadth_ of bivalve shells is variously measured. Most
authorities measure _length_, or ‘antero-posterior diameter,’ by a
straight line drawn from the extreme anterior to the extreme posterior
margin, and _breadth_ by a similar line, drawn from the umbones to a
point, not very clearly marked, on the opposite ventral margin (see
Figs. 184 and 185). Others, less correctly, reverse these terms.
_Thickness_ is measured by the extreme distance of the opposite faces
of the closed valves. As a rule, the length exceeds, and often greatly
exceeds, the thickness, but in a few cases--_e.g._ the _Cardissa_
section of _Cardium_--this is reversed.
_The periostracum._--Nearly all shells are covered, at some period of
their growth, by a _periostracum_,[351] or surface skin, which serves
the purpose of protecting the shell against the destructive effects of
the chemical action set up by water or air. It also, in some cases (see
p. 258), acts as a kind of base upon which the shell is deposited. In
old shells it is commonly worn away, especially at those parts which
are likely to become abraded.
The form and composition of the periostracum varies greatly. Sometimes
(_e.g._ _Oliva_) it is a mere transparent film, at others (_Zonites_)
it is transparent, but stout and solid. It is corneous in _Solenomya_,
covered with fine hairs in many _Helicidae_, in _Conus_, _Velutina_,
and _Cantharus_ it is thick, fibrous, and persistent; in _Trichotropis_
and some _Triton_ it is furnished with long bristles on a thick ground
(Fig. 191). In fresh-water shells it is usually rather thick, in order
to protect the shell from the erosive powers of certain kinds of water.
In some cases (_Mya_, _Anatina_) the periostracum is continued over the
siphons, so as to form a protection throughout their whole length.
[Illustration: FIG. 191.--_Triton olearium_ L., Mediterranean,
an example of a shell with a stout and hairy periostracum. × ½.]
_Erosion._--The fresh-water Mollusca generally, and marine mollusca in
a few rare cases (_Purpura_, _Littorina_) are subject to _erosion_,
or decay in the shell substance. In univalves erosion usually sets
in near the apex (Fig. 192), where the life of the shell may be
regarded as weakest, and in bivalves near the umbones. It is commonest
in old shells, and rarely occurs in the very young. So long as the
periostracum is present to protect the shell, erosion cannot set in,
but when once it has been removed the shell is liable to the chemical
changes set up in its substance by water. There is abundant evidence
to show that erosion is caused by pollution of water. Out of many
instances one must suffice. In a certain stream near Boston, U.S.,
numbers of Mollusca occurred, the shells of which were very perfect and
free from disease. Some little way down stream an alkaline manufactory
drained its refuse into the water. At and below this point for some
distance every shell was more or less eroded, most of them seriously.
Farther down, when the alkali refuse became diluted away, the shells
retained their normal condition.[352]
[Illustration: FIG. 192.--Example of an eroded fresh-water shell
(_Melania confusa_ Dohrn, Ceylon).]
A small percentage of lime in the water appears to produce erosion. The
result of some experiments by G. W. Shrubsole, in the investigation of
this point, may be tabulated as follows:[353]--
Water from Lime present Result
per gall.
River Dee, near Chester 3·00 grs. acted strongly on shells
Wrexham 4·00 grs. „ „ „
River Dee, near Llanderyel 0·53 grs. „ „ „
Trent Canal 8·33 grs. no action „
CHAPTER X
GEOGRAPHICAL DISTRIBUTION OF LAND AND FRESH-WATER MOLLUSCA--THE
PALAEARCTIC, ORIENTAL, AND AUSTRALASIAN REGIONS
The Mollusca afford specially valuable evidence on problems of
geographical distribution. This fact is largely due to their extreme
susceptibility to any change in the conditions of life. Genera which
are accustomed to live in a certain temperature and on certain food,
cannot sustain life if the temperature falls or rises beyond certain
limits, or if the required food be not forthcoming. There is therefore
a marked contrast between the Mollusca of the tropics and of the
temperate zones, while different regions in the same latitude, whether
within or without the tropics, often show great diversity in their
fauna. Every region is thus _characterised_ by its Mollusca. The
Mollusca, for instance, of Australia or of South Africa characterise
those countries quite as much as do the kangaroo and the emu, the
hartebeest and the ostrich; there is nothing like them anywhere else in
the world. In the Greater Antilles the Mollusca stand out beyond all
other forms of life as characteristic of the islands as a whole, and of
each separate island in particular.
The geographical distribution of the land and fresh-water Mollusca must
be considered quite apart from that of the marine Mollusca. The sea
offers no such serious barriers to the spread of the latter as the land
does to the spread of the former. If we were to journey to the Azores,
and turn our attention to the land-snails, we should find them almost
wholly peculiar, while amongst the sea-shells we should recognise many
as occurring also on our southern coasts, and few that were different
from those of the Mediterranean. The marine Mollusca of the Sandwich
Islands, in spite of the enormous intervening distance, are not very
different from those of Natal, but the land Mollusca of the two
countries are as widely different as is possible to imagine.
Land Mollusca are, as has been remarked, fettered to the soil.
Quadrupeds, birds, fishes, and reptiles are provided with organs of
motion which enable them to overpass barriers of various kinds. Even
plants, although themselves incapable of motion, may be conveyed in
every direction by means of seeds, which are either wafted by the wind
or adhere to the skin of animals. But the Mollusca have no such regular
means of transport, and are, in a large number of instances, limited to
districts of a certain character of soil, or producing certain kinds of
vegetation.
The localisation, both of genera and species, occurs all over the
world. The genus _Achatinella_, which is peculiar to the Sandwich
Islands, is found there in a profusion of species. It lives in the
mountain valleys which radiate from the central ridge of each island,
and each valley is characterised by its own peculiar set of species.
The great carnivorous _Glandina_ is restricted to Central America and
the adjacent parts of the two continents, with one or two species in
Southern Europe. _Bulimus_ proper is restricted to South America;
_Achatina_ to Africa south of the Sahara; _Tornatellina_ to the
Pacific Islands; _Cochlostyla_ to the Philippines; _Cylindrella_ and
_Bulimulus_ are peculiar to the New World; _Buliminus_, _Nanina_,
_Scarabus_, and _Cassidula_ to the Old.
Extreme cases of this restriction of habitat sometimes occur. Thus
_Limnaea involuta_ is found only in a single small mountain tarn in
Ireland; _Clausilia scalaris_ along a narrow strip of limestone in
Malta; _Strophia nana_ is confined to a few square rods on an island
that is itself a mere dot in the Caribbean Sea; the genus _Camptonyx_
occurs only in the neighbourhood of Mt. Girnar, in Gujerat; and
_Lantzia_ in moss on the top of a mountain in Bourbon.
Attempts to colonise snails in strange localities have usually resulted
in failure, especially when the attempt has involved serious changes
of environment. The common _Cochlicella acuta_ of our coasts resists
all endeavours to establish it beyond a certain distance from the
sea. Snails brought from the Riviera and placed under almost similar
conditions of climate on our own southern coasts have lived for a
while, but have very rarely taken permanent root. Mr. H. W. Kew[354]
has collected a good many of these attempts to acclimatise species,
the general success of which seems to depend almost entirely on a
restoration of the old conditions of life.
At the same time there are certain species which exhibit a curiously
opposite tendency, and which seem capable of flourishing in almost
any part of the world, and under the most varied surroundings. Our
own common garden snail (_Helix aspersa_) is a striking instance of
this adaptability to new conditions. It has been established, by art
or by accident, in Nova Scotia, Maine, South Carolina, New Orleans,
California, Mexico city, Cuba, Hayti, Cayenne, Brazil, Valparaiso,
Cape Town, the Azores, St. Helena, Mauritius, Loyalty Islands, and
Australia. The great _Achatina fulica_ of East Africa has been
established first in Mauritius, and from thence has been carried to
the Seychelles and Calcutta. _Helix lactea_, a common Mediterranean
species, has been carried to Teneriffe and Montevideo; _Helix
similaris_, whose fatherland is Eastern Asia, has been transported to
Mauritius, Bourbon, West Africa, West Indies, Brazil, and Australia;
_Ennea bicolor_ (Eastern Asia) to India, Bourbon, Mauritius, West
Indies; _Stenogyra decollata_ (Mediterranean basin) to South Carolina;
_S. Goodallii_ (West Indies) to British pineries; _Helix Hortensis_ to
New Jersey. Seven common English species (_Limax gagates_, _Hyalinia
cellaria_, _H. alliaria_, _Helix aspersa_, _H. pulchella_, _Pupa
umbilicata_) have become naturalised in St. Helena,[355] and as many as
nineteen in Australia.[356]
Cases of artificial transport of this kind are readily detected; they
follow the lines of trade. The snails themselves or their ova have
been accidentally enclosed with plants or mould, or have adhered
to packing-cases, or to hay and grass used in packing. Thus they
constitute no disturbance to the general rule of the persistent
localisation of species and genera, and there is little fear that the
evidence which the geographical distribution of the Mollusca brings to
bear upon the general problems of distribution will be confused by any
intermixture of fauna naturally distinct.
_Land Mollusca: Barriers to Dispersal._--The chief natural barriers
to dispersal are extremes of temperature, the sea, mountain ranges,
and deserts. Rivers, however large, seem of little effect in checking
dispersal. There is no appreciable difference between the land Mollusca
north and south of the Ganges, or north and south of the Amazon. Living
snails, or their ova, are no doubt transported from one bank to another
on floating débris of various kinds. The barrier offered by the sea is
obvious, and at first sight appears insurmountable; but the facts with
regard to oceanic groups of islands like the Azores and Canaries (see
p. 297) show that even a stretch of salt water many hundred miles in
breadth may be ineffectual in preventing the dispersal of Mollusca.
Mountain ranges, provided they are too high to be scaled, and too
long to be turned in flank, offer a far more effective barrier than
the sea. Every thousand feet upward means a fall of so many degrees
in the mean temperature, and a change, more or less marked, in the
character of the vegetation. There is generally, too, a considerable
difference in the nature of the climate on the two sides of a great
mountain range, one side being often arid and cold, the other rainy and
warm. The combined effect of these influences is, as a rule, decisive
against the dispersal of Mollusca. Thus the Helices of California are
almost entirely peculiar; one or two intruders from states farther east
have succeeded in threading their way through the deep valleys into
the Pacific provinces, but not a single genuine Californian species
has been able to scale the heights of the Cascade Mountains. The land
Mollusca of India are numbered by hundreds; not one penetrates north
of the Himalayas. According to Mr. Nevill,[357] the change from the
Indo-Malayan to the so-called European molluscan fauna at the northern
watershed of the Kashmir valley is most abrupt and distinct; in two
days’ march northward, every species is different. Ranges of inferior
altitude, such as the Pyrenees, the Carpathians, or the Alleghanies,
may be turned in flank as well as scaled, and we find no such marked
contrast between the Mollusca on their opposite sides.
The most effective barrier of all, however, is a desert. Its scorching
heat, combined with the absence of water and of vegetable life, check
dispersal as nothing else can. The distribution of the Mollusca of the
Palaearctic Region is an excellent instance of this. Their southern
limit is the great desert which stretches, with scarcely a break,
from the west coast of Africa to the extreme east coast of Asia. The
Mediterranean offers no effectual barrier; shells of southern Europe
are found in profusion in Morocco, Tunis, and Egypt, while all through
Siberia to the extreme of Kamschatka the same types, and even the same
species, of Mollusca occur.
A detailed examination of the means, other than voluntary, by which
Mollusca are transported from one place to another hardly comes within
the scope of this work. Ocean currents, rivers, floods, cyclonic
storms of wind, birds, and even beetles and frogs, play a part, more
or less considerable, in carrying living Mollusca or their ova, either
separately or in connexion with floating débris of every kind, to a
distance from their native home. Accidental locomotion, of one or
other of these kinds, combined with the well-known tenacity of life
in many species (p. 37), may have contributed to enlarge the area of
distribution in many cases, especially in the tropics, where the forces
of nature are more vigorous than in our latitudes. The ease with which
species are accidentally spread by man increases the probability of
such cases occurring without the intervention of human agency, and
numbers of instances may be collected of their actual occurrence.[358]
A point, however, which more concerns us here is to remark on the
exceedingly wide distribution of the prevailing forms of fresh-water
Mollusca. It might have been expected that the area of distribution in
the fresh-water forms would be greatly restricted, since they cannot
migrate across the land from one piece of water to another, and since
the barriers between pond and pond, lake and lake, and one river system
and another are, as far as they are concerned, all but insuperable. We
might have expected, therefore, as Darwin and Wallace have remarked, to
find a great multiplicity of species confined to very restricted areas,
since the possibility of communication with the parent stock appears,
in any given case, to be so exceedingly remote.
As is well known, the exact reverse occurs. The range, not merely of
genera, but even of individual species, is astonishingly wide. This is
especially the case with regard to the Pulmonata and Pelecypoda. The
genera _Limnaea_, _Planorbis_, _Physa_, _Ancylus_, _Unio_, and _Cyclas_
are world-wide. Out of about ten genera of fresh-water Mollusca in
New Zealand, one of the most isolated districts known, only one is
peculiar. In South Africa and the Antilles no genus is peculiar. In the
latter case, this fact is remarkable, when we consider that the same
sub-region has at least ten peculiar genera of operculate land Mollusca
alone.
To give a few instances of the distribution of particular species:--
_Limnaea stagnalis_ L. occurs in the whole of Europe, and northern
Asia to Amoorland, Turkestan, Afghanistan, North Persia, and Kashmir;
Greenland, North America from the Atlantic to the Pacific, and
from North Canada and British Columbia as far south as Texas. The
distribution of _L. peregra_ Müll., _L. truncatula_ Müll., and _L.
palustris_ Müll, is almost equally wide.
_Planorbis albus_ occurs in the whole of Europe, and northern Asia to
Amoorland, Kamschatka, and Japan; Turkestan, the Altai-Baikal district,
Alaska and Greenland, North Canada, and the whole of eastern North
America.
The distribution of _Anodonta anatina_ L., _Cyclas cornea_ L., and
_Pisidium pusillum_ Gmel. is almost equally wide.
It is evident that the accidental means of transport mentioned above
are insufficient to account for the facts as we find them; we are
therefore compelled to seek for further explanation. Anything in the
nature of a current furnishes a ready means of transport for Mollusca
which have obtained a footing in the upper waters of a river, and there
is no difficulty in imagining the gradual spread of species, through
the agency of floods or otherwise, over a whole river system, when
once established at any point upon it. The feeble clinging power of
newly-hatched _Limnaea_ has often been noticed as contributing to the
chances of their range of distribution becoming extended. Fresh-water
Mollusca, too, or their ova, are exceedingly likely, from their
extreme abundance, to be transported by water-birds, which fly without
alighting from one piece of water to another. Again, the isolation
of one river system from another is, in many instances, by no means
well marked or permanent, and a very slight alteration of level will
frequently have the effect of diverting the supplies of one watershed
into another. When we know what enormous oscillations in level have
taken place over practically the whole surface of the globe, we can
recognise the probability that the whole river system of the earth has
been mixed up and reconstructed again and again, with a very thorough
blending of adjacent fauna.
It is possible that the very uniform conditions under which fresh-water
Mollusca live may have something to do with the uniformity of their
distribution and the comparative sameness in their development. There
can scarcely be any question that the environments of fresh-water
species are in themselves less varied and less liable to fluctuation
than those of species whose home is the land. Water is very like
water, all the world over; it may be running or motionless, warm or
cold, clear or muddy, but the general tendency is for it to be free
from extremes of any kind. Even if the surface water of a lake or
river freezes, or becomes unusually hot, there is generally plenty of
water at a lower stratum which maintains a less extreme temperature,
and to which creatures can retire on the first symptoms of a change.
From this two results will follow. Not only will the inhabitants of a
piece of water not be inclined to vary much from the type, since their
whole surroundings, food, etc., continue very much the same, but,
if transported by any accident or cataclysm elsewhere, they will be
exceedingly likely to arrive at a place which closely resembles their
former home in all essentials. Thus the tendency for new types to be
formed would be constantly checked, or rather would very seldom arise.
Mr. Belt, while recognising the importance of changes of level as
affecting the distribution of fresh-water species, appears to regard
the operations of such changes from a rather different point of view
to that described above. “I think it probable,” he writes,[359] “that
the variation of fresh-water species of animals and plants has been
constantly checked by the want of continuity of lakes and rivers in
time and space. In the great oscillation of the surface of the earth,
of which geologists find so many proofs, every fresh-water area has
again and again been destroyed.... Thus species of restricted range
were always exposed to destruction, because their habitat was temporary
and their retreat impossible, and only families of wide distribution
could be preserved.”
* * * * *
The terrestrial surface of the globe has been divided, as indicating
the facts of geographical distribution, into six regions--the
Palaearctic, Oriental, Australasian, Ethiopian, Nearctic, and
Neotropical. To these is sometimes added a seventh, the Neantarctic,
consisting of Chili and Patagonia (and certain islands of the south
Atlantic); but since the Mollusca of Chili unmistakably form a part
of the Neotropical fauna, it seems hardly worth while to recognise a
separate region for those of the extreme south of South America, which
have no peculiar characteristics.
In certain points the exact limits of these regions, as indicated by
the Mollusca, will probably not correspond to those which are marked
out by other zoological classes. Wallace’s line, for instance, does not
exist, as far as the Mollusca are concerned.
These regions may be further subdivided into sub-regions, thus:--
Regions Sub-regions
{ Septentrional
=Palaearctic= { Mediterranean
{ Central Asiatic
=Oriental= { Indo-Malay
{ Chinese
{ Papuan
=Australasian= { Australian
{ Polynesian
{ Central African
=Ethiopian= { South African
{ Malagasy
{ American
=Nearctic= { Californian
{ Antillean
{ Central American
=Neotropical= { Colombian
{ Brazilian
{ Chilian
=A. The Palaearctic Region=
The southern boundary of this region is the northern limit of the
African Sahara, the Mediterranean forming no break whatever in its
continuity. In Asia this boundary is less well marked, but roughly
corresponds to the southernmost of the vast ranges of mountains
which border the great tablelands of central Asia. Across Africa the
line of desert is well defined; but in the north-east, as the desert
approaches more nearly to the sea, the African extent of the region
is correspondingly narrowed until it becomes little more than a strip
of coast land, scarcely widening even in Lower Egypt. On the Morocco
coast, Palaearctic land forms penetrate as far south as Cape Nun.[360]
At its eastern extremity the line becomes less well defined, but
probably proceeds along the snowy mountains west of Setchouan, the
Pe-ling and Tan-sia-shan ranges, so as to include all the high ground
of Thibet and of the upper waters of the Hoang-ho, and ultimately
reaches its eastern limit at some point on the shores of the Sea of
Japan.
The region thus includes all Europe, Africa north of the Sahara, with
the Atlantic islands (the Azores, Canaries, etc.), North Arabia,
Asiatic Turkey, the greater part of Persia, Afghanistan, Thibet, all
Asiatic Russia, and a very large portion of the Chinese empire.
The principal characteristics of the region as a whole are:--
(1) The rich development of _Helix_, _Arion_, _Limax_, _Buliminus_, and
_Clausilia_.
(2) The comparative absence of land operculates (see map,
_frontispiece_).
(3) The uniform character of the fresh-water fauna.
It is in the southern portion of the region that _Helix_ (in the
sub-genera _Macularia_, _Iberus_, _Pomatia_, and _Xerophila_) and
_Buliminus_ (_Zebrina_, _Chondrula_, _Ena_) attain their maximum.
In the north, _Fruticicola_ is the characteristic group; in the
mountainous districts of the south-east, _Campylaea_, with _Clausilia_.
The Arionidae have their headquarters in the damp and warm regions
of western Europe, but are rare in the south. They only approach the
Mediterranean coast in Algeria, near Gibraltar, and in the region
between the base of the Pyrenees and the Maritime Alps, and are very
poor in species throughout Italy and Sardinia. They are absent from
almost the whole of northern Africa, the Mediterranean islands (except
Sardinia), the whole Balkan district, the Crimea, Caucasus, and western
Asia.[361]
The uniformity of the fresh-water fauna is disturbed only in the
extreme south. A few species of _Melanopsis_, with _Neritina_, occur
in southern Spain and Austria, Galicia, and southern Russia, while a
_Melania_ or two (absent from Spain) penetrate the south-eastern parts
of Europe as far as Germany. _Cyrena_ begins to replace _Cyclas_ in
southern Russia and the Caucasus.
The Palaearctic region falls into three sub-regions:--
(1) The =Northern or Septentrional Sub-region=, _i.e._ the district
north of the line formed by the Pyrenees,[362] Alps, Carpathians, and
which, passing to the northward of the Aralo-Caspian district, follows
the great central mountain range of Asia until it reaches the Sea of
Japan, perhaps somewhere in the neighbourhood of Vladivostok.
(2) The =Mediterranean Sub-region=, _i.e._ the countries bordering
on the Mediterranean, the Black and Caspian Seas, with the Atlantic
Islands.
(3) The =Central Asiatic Sub-region=, _i.e._ Turkestan, Afghanistan,
Thibet, and probably the districts of Mongolia and Manchuria.[363]
(1) The =Septentrional Sub-region= has been divided by some writers
into two provinces, the European and the Siberian. There seems, on
the whole, but little occasion to separate off northern Asia, the
characteristic of which is, as will be seen below, rather the gradual
disappearance, as we proceed eastward, of European species and genera,
than the development of any new and peculiar groups. The remarkable
fauna of Lake Baikal stands apart, not only from European, but also
from the Siberian types occurring in its immediate neighbourhood.
On the whole, the Septentrional Sub-region is poor in species
except those which inhabit fresh water. This fact is probably due
to the extreme vicissitudes of temperature which prevail, and it is
interesting to notice that the number of land Mollusca appears to touch
its lowest point in districts where the annual range of temperature is
greatest. On the other hand, in the western portions of the region,
where the climate is moist and temperature more equable, the Mollusca
are considerably more abundant and varied.
The line which separates the Septentrional from the Mediterranean
Sub-region must of necessity be very roughly drawn, and stragglers from
the south will be found to make their way northward, and _vice versâ_,
under favouring circumstances of temperature and geological formation.
Jordan has noticed[364] that species which in southern countries are
not confined to any particular quality of soil are in more northern
latitudes found only on limestone, which absorbs more heat than other
formations. Conversely, the higher elevations of the Alps, Pyrenees,
and even Carpathians are like islands in a sea, and support a
thoroughly northern fauna, quite strange to that of the plains below.
Thus _Helix harpa_ Say, a completely boreal shell, which is at home in
Canada, Sweden, Lapland, and the Amoor district, is found on the Riffel
Alp, at a height of 6000 feet.[365] _Vertigo arctica_ Wall., a species
abundant in Lapland, North Siberia, Iceland, and Greenland, occurs on
the high Alps of the Tyrol.
_Circumpolar Species._--A certain number of species are common to
the extreme north both of the Palaearctic and Nearctic regions, and
are, in fact, circumpolar. The number of these species, however, is
so small, not exceeding about 40 species (= 16 genera), that it seems
hardly worth while creating a special sub-region for their reception,
particularly as no genus is peculiar. At the same time the fact is
instructive as illustrating the close connexion of the northern
districts of the two regions, a connexion which was no doubt more
intimate in recent geological times than it is now.
The circumpolar genera are as follows. The list decisively sets forth
the superior hardiness of the fresh-water as compared with the land
genera:--
Valvata 1 sp.
Bithynia 1 „
Vitrina 1 „
Hyalinia 4 „
Helix 2 „
Patula 2 „
Pupa 3 „
Cionella 1 „
Succinea 1 „
Limnaea 7 „
Planorbis 5 „
Aplecta 1 „
Physa 1 „
Anodonta 1 „
Unio 1 „
Pisidium 1 „
_Great Britain._--There are in all about 130 species--83 land, 46
fresh-water; _Limnaea involuta_ (mountain tarn near Killarney) appears
to be the only peculiar species. There are 11 _Hyalinia_, 5 _Arion_,
and 25 _Helix_, the latter belonging principally to the sub-genera
_Xerophila_, _Tachea_, _Trichia_, and _Fruticicola_. Three _Testacella_
are probably not indigenous, but are now so well established as to
reckon in the total. Of the four _Clausilia_ two reach Ireland and
one Scotland; two do not occur north of the Forth. There are only
two land operculates, one of which (_Cyclostoma elegans_) occurs in
Ireland but not in Scotland, while the other (_Acicula lineata_)
reaches the southern counties of Scotland. Several species, _e.g._
_Helix pomatia_, _H. obvoluta_, _H. revelata_, _H. cartusiana_, _H.
pisana_, _Buliminus montanus_, are restricted to the more southern or
western counties; _Geomalacus maculosus_ is confined to a district in
south-western Ireland.
The Pleistocene beds of East Anglia contain a number of species now
extinct in these islands, whose occurrence appears to indicate a warmer
climate than the present. Such are _Helix ruderata_, _H. fruticum_,
_H. incarnata_, _Clausilia pumila_, _Unio littoralis_, _Hydrobia
marginata_, and _Corbicula fluminalis_.
_Scandinavian Peninsula._--From Norway 121 species in all are recorded,
and 148 from Sweden. The milder climate of Norway allows many species
to reach a considerably higher latitude than in Sweden, thus in Sweden
_Limax maximus_ only reaches 62°, but in Norway 66° 50´. Similarly
_Arion hortensis_ and _Balea perversa_ only reach 63° and 61°
respectively in Sweden, but in Norway are found as far north as 69° and
67° 50´. _Clausilia_ is represented by 9 species in southern Norway;
one of these is found north of the Arctic circle. There are 4 _Pupa_,
9 _Vertigo_, and 11 _Hyalinia_, but _Helix_ dwindles to 14, 9 of which
occur north of the Arctic circle. No land operculates are found;
_Cyclostoma elegans_, however, occurs in Jutland and Zealand, which
practically form a part of this district.
_Iceland._--Eleven species, all Scandinavian, occur. These are _Arion_
2, _Limax_ 1, _Helix_ 2 (_arbustorum_ L. and _hortensis_ Müll., the
latter being found only on the warmer southern coast), _Limnaea_ 1,
_Planorbis_ 1, _Pisidium_ 4.
_France._--The northern, central, and eastern districts belong to
this sub-region, while the southern and western, in which an entirely
new element occurs and many northern forms disappear, belong to the
Mediterranean. Thus, for instance, _Helix pomatia_ L., _H. incarnata_
Müll., _H. fruticum_ Müll., _H. cantiana_ Mont., _H. strigella_ Drap.,
_H. rufescens_ Penn., _H. plebeia_ Drap., are not found in southern
France. No detailed enumeration of species is at present possible,
the efforts of a large number of the leading French authorities
being directed to indiscriminate species-making rather than to the
careful comparison of allied forms. Perhaps the principal difference
between the Mollusca of northern France and those of our own islands
is the occurrence of two species of _Pomatias_. In the more elevated
districts of eastern France (the Vosges, Jura, western Alps), a certain
number of species occur which are confined to the high grounds of
south central Europe. Among these are _Helix holoserica_ Stud.,
_H. personata_ Lam., _H. bidens_ Chem., _H. depilata_ Drap., _H.
cobresiana_ Alt., _H. alpina_ Faure.
The Pleistocene deposits of the valley of the Somme tell the same
tale as those of eastern England, containing as they do species
and even genera whose northern range is now much more limited. The
Eocene fossils from the Paris beds show most remarkable relationships
to genera now existing in the West Indies and Central America.
Others again indicate affinities with India. Thus we find _Ceres_,
_Megalomastoma_, and _Tudora_ by the side of _Leptopoma_, _Faunus_, and
_Paludomus_.
_Germany._--The Mollusca of the plains of northern Germany are
few and not striking, and exhibit little difference from those of
our own islands. In the mountainous districts of the south and
south-east, a number of new forms occur, amongst which are 3 species
of _Daudebardia_, a remarkable carnivorous form, with the general
appearance of a _Vitrina_; 24 of _Clausilia_, many _Pupa_, several
_Buliminus_, 3 of the _Campylaea_ group of _Helix_, stragglers from
the Italo-Dalmatian fauna, and 1 of _Zonites_ proper. Our familiar
_Helix aspersa_ is entirely absent from Germany. There are only 4 land
operculates--_Pomatias_ 2, _Acicula_ 1, _Cyclostoma_ 1, all of which
occur exclusively in the south. _Bithynella_ and _Vitrella_, two minute
forms of fresh-water operculates akin to _Hydrobia_, occur throughout
the district.
[Illustration: FIG. 193.--=A=, _Daudebardia brevipes_ Fér.: _sh_,
shell; _p.o_, pulmonary orifice. (After Pfeiffer.) =B=, shell of
_D. rufa_ Pfr., S. Germany.]
_Northern Russia and Siberia._--This vast tract extends from eastern
Germany to the Amoor district. It is exceedingly poor in Mollusca, and
is chiefly characterised by the gradual disappearance, as we proceed
eastward, of European species. There are a few characteristic Siberian
Mollusca, closely allied to European forms, and in the extreme east a
new element is introduced in the appearance of types which indicate
Chinese affinities. The whole district may be regarded as bounded to
the south by a line drawn from Lemberg to Moscow, and thence to Perm;
passing south of the Ural mountains, it includes the whole basins of
the rivers Obi, Yenesei, and Lena, coinciding with the vast mountain
ranges which terminate to the north the table-land of central Asia, at
the eastern extremity of which it dips sharply southwards, so as to
include the Amoor basin and Corea.
All the larger Helices are wanting, and no land operculates occur.
_Helix arbustorum_ L., _H. nemoralis_ Müll., _H. lapicida_ L., _H.
aculeata_ Müll., and _Hyalinia nitidula_ Drap., do not appear to occur
east of the Baltic; _Arion fuscus_ Müll., _Helix strigella_ Drap.,
_Buliminus obscurus_ Müll., _Clausilia laminata_ Mont., _C. bidentata_
Bttg., _C. plicatula_ Drap., _Viviparus fasciatus_ Müll., and _Neritina
fluviatilis_ L., do not pass the Urals.
In the Obi district (West Siberia) a further batch of European species
find their easterly limit. Among these are _Helix hispida_ L.,
_Bithynia tentaculata_ L., _Vivipara vivipara_ L., _Pisidium amnicum_
Müll., and _Unio tumidus_ Retz. A few distinctly Siberian species now
appear, _e.g._ _Ancylus sibiricus_ Gerst., _Valvata sibirica_ Midd.,
and _Vitrina rugulosa_ Koch.
The following are among the European species which reach eastern
Siberia: _Hyalinia nitida_ Müll., _Succinea oblonga_ Drap., _Planorbis
vortex_ L., _spirorbis_ L., _marginatus_ Drap., _rotundatus_ Poir.,
_fontanus_ Light., _Valvata piscinalis_ Müll., _Bithynia ventricosa_
Leach, and _Anodonta variabilis_ Drap. Here first occur such
characteristic species as _Physa sibirica_ West., _P. aenigma_ West.,
_Helix pauper_ Gld., _H. Stuxbergi_ West., _H. Nordenskiöldi_ West.,
_Planorbis borealis_ Lov., _Valvata aliena_ West., _Cyclas nitida_
Cless., and _C. levinodis_ West. In the Amoor district a decided
Chinese element makes its appearance in a few hardy forms which have
penetrated northward, _e.g._ _Philomycus bilineatus_ Bens., and a few
each of the _Fruticicola_ (Chinese) and _Acusta_ groups of _Helix_. Out
of 53 species, however, enumerated from this district, as many as 33,
belonging to 18 genera, occur also in Great Britain.
_Lake Baikal._--The Mollusca of Lake Baikal exhibit distinct
characteristics of their own, which seem to indicate the long-continued
existence of the lake in its present condition. Several entirely
peculiar genera occur, which are specialised forms of _Hydrobia_,
_e.g._ _Baikalia_, _Liobaikalia_, _Gerstfeldtia_, _Dybowskia_, and
_Maackia_; _Benedictia_ alone extends to the basin of the Amoor.
_Choanomphalus_, another peculiar and ultra-dextral (p. 250) genus
belonging to the Limnaeidae, appears to be related to the West American
_Carinifex_.
(2) The =Mediterranean Sub-region= is divided into four provinces:
(_a_) The Mediterranean province proper; (_b_) the Pontic; (_c_) the
Caucasian; and (_d_) the Atlantidean province.
(_a_) The _Mediterranean province proper_ is best considered by further
subdividing it, with Fischer and others, into separate districts, each
of which has certain peculiar characteristics.
(i) The _Hispano-Algerian_ district includes the greater part of
the Iberian peninsula, the Balearic Islands, and northern Africa
from Morocco to Tunis. The extreme western parts of these districts,
including West Morocco, Portugal, Asturias, and south-west France,
under the influence of the moist climate caused by the Atlantic, show
some peculiar features which, in the view of some, are sufficient to
justify their separation from the rest of the Hispano-Algerian portion.
Among these is a marked development of the slugs, _Testacella_,
_Arion_, and _Geomalacus_, the latter of which occurs even in
south-western Ireland.
[Illustration: FIG. 194.--=A=, _Parmacella Valenciensii_ W. and
B. × ⅔. (After Moquin-Tandon.) =A´=, shell of the same, natural
size.]
_Spain._--The principal features are the development of the
_Macularia_, _Iberus_, and _Gonostoma_ groups of _Helix_, and the
occurrence of the remarkable slug _Parmacella_, which is found in
many other parts of the sub-region, and extends eastward as far as
Afghanistan. _Clausilia_ has but few species, mostly in the north.
There are four species of land operculates, one of which is referred
to a genus (_Tudora_) now living only in the West Indies, but which
occurs in the Eocene fossils of the Paris basin. In the south there are
several species of _Melanopsis_ and _Neritina_.
The _States of Northern Africa_ have a thoroughly Mediterranean fauna,
whose facies on the whole shows rather more affinity to Spain than to
Sicily. The Helices of Morocco and Algeria belong to the same groups as
those of southern Spain. Many are of a dead white colour, the better
to resist the scorching effect of the sun. _Ferussacia_ is abundant,
_Geomalacus_ and _Parmacella_ are represented by a single species
each, and there is one _Clausilia_. According to Kobelt,[366] the
original land connexion between southern Spain and Morocco must have
been much more extensive than is usually assumed, and probably reached
at least to the meridian of Oran and Cartagena. The Mollusca of Oran
and Cartagena are, according to him, much more closely related than
those of Oran and Tangier, or those of Cartagena and Gibraltar, but at
Cartagena some species, which are characteristic of the Mediterranean
coasts from Syria westward, disappear, are absent from the rest of
Spain and from Morocco, but reappear on the south-western coasts of
France. These species may possibly have pushed along that arm of the
sea which, when the Straits of Gibraltar were closed as far as the
latitude of Oran and Cartagena, united in comparatively recent times
the Bay of Biscay with the Gulf of Lions.
The following genera, which do not occur in Spain, have probably spread
into northern Africa as far as Algeria, _via_ Sicily and Tunis, namely,
_Glandina_ (1 sp.), _Daudebardia_ (1 sp.), _Pomatias_ (2 sp.). Tunis
shows strong traces of Sicilian influence, and Kobelt found a colony of
snails, of Sicilian affinities, as far west as Tetuan.
_The Sahara._--The Algerian Sahara contains, in many places, a
sub-fossil Molluscan fauna which appears to show that the district
has, in quite recent times, undergone a gradual desiccation. The
species are mainly fresh-water, including _Melania_, _Melanopsis_,
and _Corbicula_, with here and there valves of _Cardium edule_, and
indicate, on the whole, an affinity with recent Egyptian, rather than
North African species. It is probable that a vast series of _étangs_,
or brackish-water lakes, once stretched along this region, and were
ultimately connected with the sea somewhere between Tunis and Egypt.
[Illustration: FIG. 195.--Characteristic shells of S. France:
=A=, _Helix_ (_Macularia_) _niciensis_ Fér.; =B=, _Leucochroa
candidissima_ Drap.]
(ii) _Southern France._--The southern portion of France bordering on
the Mediterranean contains many species, especially of _Helix_, which
do not occur in the centre and north. Amongst these are--
Leucochroa candidissima Drap.
Hyalinia olivetorum Gmel.
Zonites algirus L.
Helix rangiana Desh.
„ serpentina Fér.
„ niciensis Fér.
„ splendida Drap.
„ vermiculata Müll.
„ melanostoma Drap.
„ aperta Born.
„ ciliata Ven.
„ explanata Müll.
„ apicina Lam.
„ cespitum Drap.
„ Terverii Mich.
„ pyramidata Drap.
„ trochoides Poir.
Ferussacia folliculus Gron.
Rumina decollata L.
Pupa megacheilos C. and J.
Several species of fresh-water _Hydrobia_ (_Bithynella_) occur. The
district, on the whole, unites certain characteristics derived from
northern Italy with those of eastern Spain.
(iii) The _Italo-Dalmatian_ district includes Italy and the
neighbouring islands (Corsica, Sardinia, Sicily, Malta), and the
regions at the head and north-eastern shores of the Adriatic
(Carinthia, Carniola, Croatia, and Dalmatia), the line which separates
these latter districts from the fauna of southern Austria, Bosnia, and
Servia being very difficult to define.
[Illustration: FIG. 196.--_Helix_ (_Pomatia_) _aperta_ L., S.
France, showing epiphragm.]
[Illustration: FIG. 197.--_Helix_ (_Campylaea_) _zonata_ Stud.,
Piedmont.]
[Illustration: FIG. 198.--_Helix_ (_Iberus_) _strigata_ Müll.,
Florence.]
_Italy_, with the neighbouring islands, has a rich molluscan fauna.
In the sub-Alpine districts of northern Italy the prominent _Helix_
groups are _Campylaea_, _Pomatia_, and _Anchistoma_, which in the south
are generally replaced by _Iberus_, which here attains its maximum
development. Large _Hyalinia_ are abundant in the north, and _Pomatias_
and _Clausilia_ are frequent all along the Apennines. _Sicily_ has
about 250 species, half of which are peculiar. Helices of the _Iberus_
type abound, but _Campylaea_ is reduced to two species. Many peculiar
forms of _Clausilia_ occur, especially a latticed type of great beauty.
_Ferussacia_ and _Pupa_ are well represented, and there are one each
of _Glandina_ and _Daudebardia_.
_Dalmatia_ and the adjacent districts are chiefly remarkable for the
rich development of _Clausilia_, which here attains its maximum (nearly
100 species). The _Campylaea_ section of _Helix_ is represented by its
handsomest forms, many of which are studded with short hairs. Here too
is the headquarters of _Zonites_ proper, which stretches westward as
far as Provence, and eastward to Asia Minor; and also of the single
European _Glandina_, which has a similar eastward range, but spreads
westward through Italy and Sicily to Algeria, not occurring in southern
France. The land operculates are chiefly represented by _Pomatias_, and
among the fresh-water operculates are a _Melania_ and a _Lithoglyphus_,
the latter having probably spread from the basin of the Danube.
[Illustration: FIG. 199.--=A=, _Clausilia crassicosta_ Ben.,
Sicily; =B=, _Clausilia macarana_ Zieg., Dalmatia; =B´=,
clausilium of same.]
(iv) The _Egypto-Syrian_ district extends along the south-eastern
shores of the Mediterranean from Tripoli to North Syria, and eastward
to the Euphrates valley. Lower Egypt alone belongs to this portion,
the fauna of Upper Egypt being of an entirely tropical character, and
belonging to the Ethiopian Region.