The shell is very different from that of a typical head-bearing mollusc, for it generally consists of two glassy, semitransparent plates, situated dorsally and ventrally respectively on the body of the animal, with an opening for the protrusion of the body, and others at the sides for processes of the mantle; and it terminates behind in one or three pointed processes. Sometimes, however, its form is conical or spiral, with or without an operculum. We append illustrations of a few pteropods, selecting for our purpose species that have been found in the Atlantic.
It will have been noticed from the above short description that the pteropod is very unlike the typical Cephalophore as outlined in our general remarks on the group, especially in the symmetrical form of both body and shell and in the total or almost total absence of the foot; and this distinction is so marked that the pteropods are often separated from all the other Cephalophora into a class by themselves, while all the remainder are placed in a separate extensive class called the Gasteropoda, because they creep on the ventral surface of the body, the term signifying stomach-footed.
These gasteropods are divided into four orders: the Nucleobranchiata, in which the respiratory and digestive organs form a nucleus on the posterior part of the back; the Opisthobranchiata, with gills more or less exposed towards the rear of the body; the Pulmonifera, or lung-breathing order; and the Prosobranchiata, in which the gills are situated in advance of the heart. The third order includes all the land snails and slugs, and does not therefore fall within the scope of our work; but the remaining three consist either exclusively or principally of marine species, and will be dealt with in the order in which they are named.
The Nucleobranchs are not really gasteropods in the strictest sense of the term, for they do not creep along by means of their foot, but all swim freely in the open ocean, always at the surface, and sometimes adhere to floating weed by means of a sucker. In fact, the foot of these creatures is greatly modified in accordance with their habits, one part being often expanded into a ventral swimming fin, and provided with a sucking-disc for adhesion, and another produced into a posterior fin for locomotion.
Like the pteropods, the nucleobranchs are purely pelagic, so that we can hardly expect to meet with a specimen on or near the shore; and thus we shall content ourselves with a brief notice of their general characters.
The shell is very variable in size and form, and sometimes even entirely absent. Large-bodied species often possess but a very small shell, while some are able to entirely retract themselves and close the mouth of the shell by an operculum. These animals are generally provided with a large cylindrical proboscis, and the tongue has recurved teeth. The body is usually very transparent, often so much so that the blood may be seen circulating within it, and the nervous system is much more perfectly developed than in the pteropods. The eyes, too, are perfectly formed.
The presence of special breathing organs may seem to be superfluous in such delicate and soft-bodied creatures as these, for it may be supposed that all the oxygen required could be absorbed directly from the water through their soft structures, as is really the case with many aquatic creatures; and as a matter of fact some of the nucleobranchs possess no gills, but others have these organs fully formed.
Passing now to the true gasteropods, we shall first consider the Opisthobranchs, which are commonly known as Sea Slugs and Sea Lemons. Some of these have no shell at all, and even where one exists it is very rudimentary, usually very small and thin, and concealed within the mantle. The gills are either branched and tree-like, or are composed of tufts or bundles of filaments; and, as the name of the order implies, are situated towards the posterior part of the body. They are also retractile, and when the animal is alarmed it will conceal its gills, thus reducing its body to a shapeless, slimy mass, inviting neither to sight nor to touch.
The sea slugs are principally animal feeders, subsisting on small crustaceans, other molluscs, &c.; the food being first reduced by the rasping action of the teeth, and then masticated in a gizzard which is provided internally with horny spines or hard, shelly plates.
It will not be necessary to enumerate all the different families of this order, especially as the species are mostly to be found beyond the tide-marks, and are therefore obtained only with the aid of the dredge; but we shall describe a few of the British species with a view of showing the general characteristics of the animals.