CHAPTER XIV 



THE ANATOMY AND PHYSIOLOGY OF MOLLUSCA: 

 ILLUSTRATED BY THE SNAIL 



General Form of the Body. — MoUusks are non-segmented 

 animals; Cuvier spoke of them as " massive " animals, since 

 their form is usual!}' short and stout in strong contrast with 

 that of the elongatetl worms. They have a right and a left 

 side and a dorsal and ventral surface, iDut in manj^ mollusks, 

 as in the snail, the bodj^ is coik^d into a close spiral, so that the 

 primitive bilateral sjinmetrj- is obscured. In view of the fact 

 that of the three main groups of the mollusks — lamelli- 

 branchs, gasteropods, and cephalopods — two are almost 

 exclusi^'ely S3-mmc>trical and the third (gasteropods) con- 

 tains a great number of sjmimetrical forms, we maj- conclude 

 that l)ilateral symmetrj' is the primitive condition for mollusks 

 and that the spiral of most gasteropods is a special and recent 

 modification. Such a spiral shell was prolxihlj- originallj' 

 a conical cap over the body. As this cap was built up at the 

 edge to accommotlate the increasing bulk of the growing young, 

 it elongated until finally it formed a long, unwieldy cone. The 

 most convenient and compact form for storing such a body is 

 a spiral or helix. This is the condition of the shell of most 

 gasteropods ; it is well illustrated by the common snail. 



The soft body of the mollusk is usually sujiported and pro- 

 tected by a shell (Fig. 1 92). This is formed from the skin, here 

 called "mantle," whose glands pour out a secretion that 



198 



