CHAPTER XII 



PHYLUM MOLLUSCA 



II 



The Phylum Mollusc a (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. 

 180), and squids (Fig. 191) 

 do not appear at first sight 

 to have much in common, 

 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 

 mud, and in the squid (III, 4) 



In each 

 there is a space called the 

 mantle cavity (Fig. 172, j) 

 between the main body and 

 an enclosing envelope, the 

 mantle. The anus (2) opens 

 into the mantle cavity. 



Ill 



Fig. 172. — Diagrams of three types of 

 mollusks, — I, a Prosobranch Gastropod, for Seizing prey 

 II, a Lamellibranch, and III, a Cephalo- 

 pod, to show the form of the foot and 

 its regions and the relations of the vis- 

 ceral hump to the antero-posterior and 

 dorso-ventral axes. A, anterior surface; 

 D, dorsal surface; P, posterior surface; 

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

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

 and MacBride, after Lankester.) 



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