CLASSIFICATION 



CHAP. 



the foot. Snails, slugs, sea-hares, whelks, periwinkles, and coats- 

 of-mail or chitons are examples of this Order. 



The ScapJiopoda 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 Bentaliiim 

 or tusk-shell of our coasts is a representative of this Order. 



Fig. 1. — Examples of the four 

 Orders : A, Cephalopoda ; 



B, Gasteropoda; C, Seapho- 

 poda, and D, Pelecypoda. 



A, Ommastrephes sagitta- 

 tus Lam., Naples : a, a, arms 

 surrounding the mouth; /, 

 funnel ; t, t, the two ' ten- 

 tacular ' arms. Xf. B, Buc- 

 cinum undatum L., Britain: 

 /, foot; pr, proboscis. X^. 



C, Dentalium entalis L., 

 Norway: /, foot. D, Car- 

 dium oblongum Chem., Na- 

 ples: /, foot; s, efferent or 

 anal siphon; s', efferent or 

 branchial siphon. X5. 



The Pelecypoda'^ 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 Ptero- 

 poda, whose exact position is at present not absolutely settled. 

 The Pteropoda^ 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- 



1 Also known as Lamellibranchiata, Conchifera, and Acephala. 



2 TTTepdv, wing. 



