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_. Highest of all the mollusca stand the cuttle-fishes, forming (with the _Nautilus_ and many extinct animals, such as ammonites and their allies) the great class _Cephalopoda_. The Cephalopoda, such as the cuttle-fish (_Sepia_) and the Poulp (_Octopus_), have now become familiar objects through our aquaria, where their very eccentric forms and remarkable movements naturally attract attention. To this group also belongs _Spirula_, the coiled and chambered shell of which is found so abundantly, but its soft tenant so very rarely. To it also belongs the extinct Belemnite, which was provided with a dense, conical internal shell, specimens of which found in rocks were at one time taken for thunderbolts. Of a lower grade of organization is the _Nautilus_, sole existing representative of a great group of Cephalopoda (including the ammonites and other forms) which has, with the above exception, long become entirely extinct.