The _Gasteropoda_ crawl on the flat under-surface or ‘sole’ of the foot. Snails, slugs, sea-hares, whelks, periwinkles, and coats-of-mail or chitons are examples of this Order. The _Scaphopoda_ possess a long tubular shell open at both ends; with their small and elongated foot they are supposed to dig into the mud in which they live. The common _Dentalium_ or tusk-shell of our coasts is a representative of this Order. [Illustration: FIG. 1.--Examples of the four Orders: =A=, Cephalopoda; =B=, Gasteropoda; =C=, Scaphopoda, and =D=, Pelecypoda. =A=, _Ommastrephes sagittatus_ Lam., Naples: _a_, _a_, arms surrounding the mouth; _f_, funnel; _t_, _t_, the two ‘tentacular’ arms, × ⅖. =B=, _Buccinum undatum_ L., Britain: _f_, foot; _pr_, proboscis. × ½. =C=, _Dentalium entalis_ L., Norway: _f_, foot. =D=, _Cardium oblongum_ Chem., Naples: _f_, foot; _s_, efferent or anal siphon; _s’_, efferent or branchial siphon, × ½.] The _Pelecypoda_[5] are enclosed in a bivalve shell fastened by a muscular hinge, the adjacent part of the valves being generally more or less toothed; the foot is as a rule roughly comparable to the shape of an axe-head. To these four Orders is frequently added a fifth, the _Pteropoda_, whose exact position is at present not absolutely settled. The Pteropoda[6] are ‘pelagic,’ _i.e._ they live in the open waters of the ocean, rising to the surface at night, and sinking into cooler water by day. They are provided with a pair of wing-like appendages or ‘feet,’ on each side of the head, by means of which they are enabled to swim. Some authorities regard the Pteropoda as a subdivision of Gasteropoda, others as forming a separate Order, of equivalent value to the other four. The question will be further discussed below (see chap. xv.), but for the present it will be sufficient to state that the weight of evidence appears to show that the Pteropoda are modified Gasteropoda, with special adaptations to pelagic life, and are therefore not entitled to rank as a separate Order.