CHAPTER VI. 



Branch VI. — Mollusca. 



General Characters of Mollusks.— In these animals 

 the body is soft, and usually protected by a shell which is 

 secreted by the skin or " mantle," but the body is not seg- 

 mented as in worms. They hare a so-called "foot" or 

 creeping disk, and the mouth is often armed with a ribbon- 

 like band provided with sharp teeth called the "lingual 

 ribbon.'' The heart is more like that of vertebrates than 

 any of the foregoing animals, consisting of a ventricle and 

 either one or two auricles. The nervous system is very 

 simple, consisting of three pairs of nerve-centres or ganglia 

 and thread-like nerves. There are about 20,000 living 

 and 19,000 extinct species of mollusks known. 



Classes op Mollusca. 



1. Shell bivalved LameUibranchiata. 



2. Shell univalve Cephalophora. 



2. Usually no shell, 8-10 arms. Cephalopoda. 



Class I. — Lamellibranchiata {Acephala, Bivalves, 

 Clam, Oyster, etc.). 



General Characters of Lamellibranchs. — The headless 

 mollusks are represented by the oyster, clam, mussel, 

 quohog, scallop, etc. By a study of the common clam 

 (Mya arenaria) one can obtain a fair idea of the anatomy 

 of the entire class. 



The clam is entirely protected by a pair of solid limestone shells, 

 connected by a hinge, consisting of a large tooth (in most bivalves 

 there are three teeth) and ligament (Fig. 72, C l). The shells are 



