2l6 ELEMENTARY ZOOLOGY. 



the shell, which is so often thought of as being the prin- 

 cipal mark of a mollusk. 



2 . While there are no paired legs, as in the grasshopper, 

 the body produces a thickening, quite different in shape 

 in different types, called a foot. This is seen in the crawl- 

 ing disc of the snail and in the large tentacles of the 

 squid. 



There are three important divisions or classes of the 

 branch of mollusks: 



1. The Lamellibranchiata, or clams, mussels, oysters, 

 etc., which usually have two parts to the shell. These 

 open and shut by means of a hinge worked by muscles. 

 These are the lowest mollusks. They seem degenerate 

 in some ways. 



2. Gasteropoda, or the snails, whelks, periwinkles, 

 slugs, in which the shell is in one piece. This is usually 

 of a spiral form. The animal can draw itself into the shell 

 or crawl partly out of it as its needs suggest. 



3. Cephalopoda, or the squid, the devil-fish, the nau- 

 tilus. These are the highest and most specialized mol- 

 lusks, and probably the highest invertebrates. The 

 majority of the living forms do not have an external 

 shell, though the nautilus does. They are chiefly re- 

 markable for the development of the head, which has a 

 great array of tentacles (Figs. 77 and 78) about the mouth. 

 These tentacles are believed to be a modification of the 

 foot in the other forms. 



236. How many specimens of Mollusca did you find in 

 your collecting tours (Chapter II)? To which of these 

 classes did they belong? Where were they found? What 

 did you discover of their habits? Get together all the 

 notes you have on them. 



