CHAPTER XII 



II 



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, 

 t but a closer examination 

 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. ,.- Diagrams of three typesof mud, and in the Squid (III, 4) 

 mollusks, I, a Prosobranch Gastropod, for Seizing prey. Ill each 

 II, a Lamellibranch, and III, a Cephalo- *.j iere j s a SDace called the 

 pod, to show the form of the foot and 



its regions and the relations of the vis- mantle cavity (Fig. 172, j) 

 ceral hump to the antero-posterior and between the main body and 

 dorso- ventral axes. A, anterior surface; J 



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

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

 3, mantle cavity; 4, foot. (From Shipley 

 and MacBride, after Lankester.) 



III 



mantle. The anus (2) opens 

 into the mantle cavity. 



242 



