The last class of vertebrates is, as we have seen, constituted by the fishes, which are fishes properly so called. But there are many animals which are familiarly and improperly spoken of as "Fishes," but which are even more below true fishes than whales and porpoises are above them. Thus, we hear of cuttle-fishes, and a variety of creatures are spoken of as "shell-fish," which are not in the least related to true fishes. Indeed, the many so-called "shell-fish" are not even nearly related one to another. Thus, the oyster and the lobster are both commonly thus named, but they belong respectively to two altogether distinct sub-kingdoms of the world of animals.
The oyster is an animal which belongs to a vast assemblage of species, with much variety of form and structure, which, on account of their soft bodies (whether or not enclosed in shells), are called MOLLUSCA or "Mollusks." This assemblage ranks as a sub-kingdom and contains within it at least four subordinate great groups, or "classes." All snails and whelks, with their allies, and also all cuttle-fishes, belong to the sub-kingdom of "soft animals."
Amongst the most familiar of mollusks is the common snail, which may serve as a type of the "class" of mollusks to which it belongs--the class _Gasteropoda_. The snail, with the slug, are representatives of land-forms of mollusca, but the bulk of the class and of the whole sub-kingdom are aquatic animals, such as the whelk (_Buccinum_), periwinkle (_Littorina_), limpet (_Patella_), &c. The Gasteropods generally possess spirally coiled shells (like the cowry or whelk), but some kinds have their shells in the form of simple cones--like a Chinaman's cap--as, _e.g._, the limpet. There are a few Gasteropods in which the shell consists of a series of similar segments as is the case with _Chiton_, while many are altogether naked. In some kinds the soft body is drawn out into a number of tufted processes, as in Doris and Eolis, and sometimes the body is almost worm-like, as in _Phylliroë_, or provided with a pair of ring-like lateral processes and a rudimentary shell, as in the sea-hare _Aplysia_.
Next above the Gasteropods comes a group of animals forming the class _Pteropoda_. These pteropods are small, active, oceanic, surface-swimming creatures, many of which live in delicate glass-like shells, and some of which form a large part of the food of the whalebone whale. They flit through the water by the aid of lateral processes which much resemble those before-mentioned as existing in the sea-hare. Allied to these pteropods is a curious little animal, the shell of which resembles a miniature elephant's tooth and which is named _Dentalium_.