CLASSIFICATION 



of the foot. Snails, slugs, sea-liares, whelks, periwinkles, and 

 coats-of-niail or chitons are examples of this Order. 



The Sccqjhojwda jjossess a long tubular shell open at l)oth 

 ends ; with their small and elongated foot they are supposed to 

 di(j into the mud in which they live. The connnon DeMalium 

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



Fid. 1. — Examples of the four 

 Orders : A, Cephalopoda ; 

 B, Gasteropoda ; C, Scapho- 

 poda, aud D, Pelecypoda. 



A, Ommast replies sagittatus 

 Lain., Naples : a, a, arms 

 surrounding the mouth ; ./', 

 funnel ; t, t, the two ' tentacu- 

 lar 'arms. X |. "B, Buccinuiii 

 11 n datum h., Britain: /', foot; 

 pr, proboscis, x i. C, De7i- 

 talium entalis L., Norway : f, 

 foot. D, Cardium ohlonyum, 

 Chem., Naples: /, foot; s, 

 efferent or anal siphon ; s' , 

 afferent or branchial siiihon. 



The Pclecyiiodo ^ are enclosed in a bivalve shell fastened hj a 

 muscular hinge, the adjacent part of the valves being generally 

 more or less toothed ; the foot is as a rule loughly comparal )le 

 to the shape of an axe-head. 



To these four Orders is frequently added a fifth, t]u? Pteroiwda, 

 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 

 liy day. They are provided with a pair of wing-like appendages 



' Also known as LamcHihrancliiata, Conchlfcra and Acrphala. 

 - irrepov, wing. 



