CHAPTER XIII.

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.

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.

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.

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.