CHAPTER VL 
BrAKCH VI. — MOLLUSCA. 
General Characters of Molltjsks.— 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 have a so-called foot" or 
creeping disk, and the mouth is often armed with a ribbon- 
like band provided with sharp teeth called the 'Mingual 
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 
^nd 19,000 extinct species of mollusks known. 
Class L — Lamellibranchiata {Acephala, Bivalves, 
Clam, Oyster, etc.). 
General Characters of Lamellihranchs. — 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 i;he 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 shcUs are 
Classes op Mollusca. 
1. Shell bivalved 
2. Shell univalve 
2. Usually no shell, 8-10 arms. . 
Lam eUibran chiata. 
Cephalophora. 
Cephalopoda. 
