GASTEROPODA.
We shall now consider the Gasteropoda, which is divided into four
orders. Firstly, Nucleobranchiata, animals which float on the surface of
the ocean: they are Diæcious, or in separate sexes, and the nervous
system is widely distributed in the body, the shell, in Carinaria, for
instance, covering only a very small portion of the body. The first
family of this order is Atlantidæ, of which the types are the fossil
Bellerophon and the recent Atlanta.
The second family is Firolidæ, the types of which are Carinaria and
Firola. Carinaria or glass nautilus is shaped like the bonnet-cap shell,
Pileopsis. It is as transparent as glass; and although now very common,
was formerly one of the most highly-prized shells by collectors. The
second order of Gasteropoda is Opistho-Branchiata, and is divided into
two sections, the Nudibranchiata, and the Tectibranchiata. The
Nudibranchiata have no shell except in the larva state; they mostly live
at the bottom of the sea on rocky shores, but a small number swim on the
surface. They are remarkable for their variety of form and vivid
colouring, being the most beautiful of all molluscous animals; they may
truly be called the caterpillars of the sea, for their branchiæ remind
us of the spines with which many lepidopterous larvæ are covered.
The first family is Elysiadæ, types Limapontia and Elysia.
The second is Phyllirhoidæ, type Phyllirhoe.
The third is Æolidæ, types Glaucus and Æolis.
The fourth is Tritoniadæ, types Scyllæa and Tritonia.
The fifth is Doridæ, types Idalia and Doris, the curious sea-lemon.
The first family of the second division, Tectibranchiata, is
Phyllidiadæ, types Diphyllidia and Phyllidia.
The second family is Pleurobranchidæ, types, Umbrella, in form
resembling a limpet, and Pleurobranchus.
The third family is Aplysiadæ, types, Dolabella and Aplysia.
The fourth family is Bullidæ, types, Scaphander, Acera, and Bulla.
The fifth family is Tornatellidæ, types, Tornatina and Tornatella.
The third order is the Pulmonifera, and the fourth is the
Prosobranchiata; we shall speak of them in the next chapter.
In this family we reach a group of Gasteropods much more numerous, both
in species and in special types, which respire by the aid of branchiæ,
or gills. Cuvier divides them into many orders, based chiefly upon their
respiratory organs.
The Tectibranchiata have the gills attached either to the right side of
the body or upon the back, arranged in the form of leaflets, more or
less divided, but not symmetrical, and nearly covered by the mantle.
_Bulla_ and _Aplysia_ are the two principal genera of the group, and may
be considered as the type of two small families.
The _Aplysiæ_ were known to the ancients under the name of sea-hares
(_Lepus marinus_), from some fancied resemblance to the terrestrial
hare. They were objects of profound horror, inspired either by their
singular form, or from an acrid, caustic, and inodorous liquid which
they secrete. A magic influence was attributed to them; they were
supposed, for instance, to have influence over the female heart. It is
not easy, however, to explain the evil renown acquired by an animal
which is known to be gentle and even timid. They are naked and fat,
somewhat resembling the Limnæa in their oval, elongated form, their
thickness in the dorsal region, and their posterior locomotion. Their
head, which is very indistinct, is furnished with four tentacles, the
anterior two of which are the largest, and somewhat resemble the ears of
a hare. The eyes are found at the base of the posterior tentacles. These
characters are observed in _Aplysia depilans_ (Fig. 180). _Aplysia inca_
shows also the same arrangement (Fig. 181). In this family the mollusc
is much more important from its volume than from its internal,
rudimentary, and horny shell, which is contained in the branchial
shield. In Fig. 182 we have the small and thin cartilaginous shell which
exists in the interior of the animal.
[Illustration: Fig. 180. Aplysia depilans (Lin).]
[Illustration: Fig. 181. Aplysia inca (D'Orbigny).]
[Illustration: Fig. 182. Shell of Aplysia inca.]
The Aplysiæ are found nearly in every region of the globe, not only upon
the shores of the Continent, but on every island shore. They commonly
inhabit sandy and muddy shores of small depths, or even the rocky
recesses, or under shelter of the stones which have fallen from the
cliffs. Their eggs consist of those long filaments which are discharged
in immense numbers, and which fishermen call sea-worms.
They feed upon certain algæ, with which the bottom of the sea is
covered; but they eat, also, small marine animals, such as the naked
molluscs, annelids, and crustaceans.
We are the less astonished to see the Aplysiæ so gluttonous when we
learn how liberally Nature has accorded to them organs of mastication,
trituration, and digestion. Their mouth is formed of thick and muscular
lips; a very long oesophagus or gullet succeeds, and this oesophagus
does not communicate with a single stomach, but with four--one enormous
membranous crop, an exceedingly muscular gizzard, with two accessary
pockets, one of which terminates in the form of a sac. The gizzard has
thick walls, and is furnished on the internal wall with cartilaginous
quadrangular pyramids, the summits of which intertwine. This apparatus
is intended to bruise the food when it reaches the third stomach. It is
also armed with little hooks, the curvature of which is directed towards
the entrance of the gizzard.
The genus _Bulla_ differs materially from the Aplysiæ. They have a
well-developed shell, the form of which is elegant; they are delicate in
structure; their brilliant colours, consisting of red, black, or white
bands, separated by many varied tints, cause these little molluscs to be
much sought after for ornamental collections. The shell itself is oval
or globulous, rolled up in a scroll, smooth, spotted, very thin and
fragile, with a concave spiral, umbilicate, open in all its length, with
a straight, wide, and cutting edge.
[Illustration: Figs. 183 and 184. Bulla ampulla (Linnæus).]
Obtuse at its two extremities, neither the head of the animal nor the
tentacles are very apparent. The gills are placed under the back, a
little to the right and behind; its stomach, which alone fills a great
part of the cavity of the body, presents the peculiarity, already noted
in the Aplysia, of being furnished with bony pieces, evidently intended
to grind the food.
[Illustration: Fig. 185. Bulla oblonga (Adams).]
[Illustration: Fig. 186. Bulla aspersa (Adams).]
[Illustration: Fig. 187. Bulla nebulosa (Gould).]
The Bullæ can swim with facility in deep water, but they evidently
prefer the shallows and a sandy bottom, feeding upon smaller molluscs.
They are found in every sea, but they abound chiefly in the Indian Ocean
and Oceania. Some species, however, such as _Bulla ampulla_ (Figs. 183
and 184), the shell of which is shaded grey and brown, and the
Water-drop (_Bulla hydratis_), inhabit European seas. _Bulla oblonga_
and _Bulla aspersa_ (Adams), and _Bulla nebulosa_ (Gould), represented
in Figs. 185, 186, and 187, are also well-known species.
We take leave of our little friends the Headless Mollusca or _Acephalæ_,
and direct our attention to those molluscs to which Nature has been more
generous, and furnished with a head. This head, however, is still
carried humbly; it is not yet _os sublime dedit_; it is drawn along an
inch or so from the ground, and in no respect resembles the proud and
magnificent organ which crowns and adorns the body of the greater and
more perfectly organized animals.
The organization of the Cephalous Mollusca present three principal
types, which has led to their being divided into three classes, after
their more salient characteristics of form and locomotive apparatus;
namely, _Gasteropoda_, _Pteropoda_, and _Cephalopoda_.
In the class _Gasteropoda_ (from γαστὴρ, _belly_, ποῦϛ,
gen.ποδὸϛ, _foot_) the locomotive apparatus consists of a
flattened muscular disk, placed under the belly of the animal, aided
by which it creeps. The Snail (_Helix_), the Slug (_Limax_), and the
Cowrie, (_Cyprea_), are types of this class.
In the _Pteropoda_, from πτερὸν, _wing_, and ποῦϛ, _foot_, the
locomotive apparatus assumes the form of wings, or membranous
swimming-fins, placed on each side of the neck. The _Hyalea_ and _Clio_
are types of this class.
In the _Cephalopoda_, from κεφαλὴ,, _head_, and ποῦϛ, _foot_,
the locomotive apparatus consists of arms, or tentacles, which surround
the mouth in numbers more or less considerable. The Cuttle-fish
(_Sepia_), and the Poulpes (_Octopoda_) are types of this last class.
The Molluscous Gasteropoda have the organs of respiration formed for
aerial respiration, or for respiration under water.
This physiological arrangement involves important differences in
internal organization in these molluscs, and renders it convenient to
divide them into two secondary groups; namely, _Pulmonary Gasteropods_,
which breathe in the air, and by a species of lung, and _Non-pulmonary
Gasteropods_, which breathe in the water, by means of branchiæ or
gills.