CHAPTER XII 



II 



i 



PHYLUM MOLLUSCA 



THE Phylum MOLLUSCA (Lat. mollis, soft) includes the snails, 

 slugs, clams, oysters, octopods, and nautili. They are primitively 



bilaterally symmetrical, but 

 unsegmented, and many of 

 them possess a shell of cal- 

 cium carbonate. Mussels 

 (Fig. 1 73), clams, snails (Fig. 

 1 80), and squids (Fig. 191) 

 do not appear at first sight 

 to have much in common, 

 p but a closer examination 

 '-3 reveals several structures 

 possessed by all. One of 

 these is an organ called the 

 foot, which in the snail (Fig. 

 172, I, 4) is usually used 

 for creeping over surfaces, 

 in the clam (II, 4) gener- 

 ally for plowing through the 



FIG. 172. - Diagrams of three types of m ud,and in the squid (III, 4} 



mollusks, I, a Prosobranch Gastropod, for seizing prey. In each 



II, a Lamellibranch, and III, a Cephalo- ,1 



pod, to show the form of the foot and there 1S a S P aCe called the 



its regions and the relations of the vis- mantle Cdvitv (Fig. 172 ?) 



ceral hump to the antero-posterior and u . , , 



dorso-ventral axes. A, anterior surface; between the mam body and 



D, dorsal surface; P, posterior surface; an enclosing envelope, the 

 V, ventral surface; /, mouth; 2, anus; ,, m, 



3, mantle cavity ; 4, foot. (From Shipley mant ^- NlC (WHS (2) Opens 



and MacBride, after Lankester.) into the mantle cavity. 



242 



III 



'.--3 



