XI. MUSSELS, CLAMS, OYSTERS, SNAILS, SQUIDS 

 Branch X. — Mollusca {mollis, soft) 



The mollusks are very widely distributed and some of 

 them form an important source of food supply. They are 

 found in the sea, on the land, and in fresh water. Most 

 of them possess very soft bodies which are, in the major- 

 ity of cases, protected by shells of carbonate of lime. 

 Although the branch, Mollusca, is divided into several 

 classes, we shall discuss only those containing the more 

 familiar forms, represented by the clam, snail, and squid 

 respectively. 



An Example of the Branch — the River Mussel 



The external. features and structure of the shell. — The 

 shell of the mussel is composed of two similar, right and 

 left pieces, or valves. In its natural position, the shell 

 is held vertically with the thin, or ventral edges buried 

 in the mud and the broad, or dorsal edges up. The 

 dorsal edges are joined for some distance by an elastic 

 band, or ligament, the h'nge ligament. The broad round 

 end of the shell is antei-ior and the more pointed end is 

 posterior (Fig. .53). Between the dorsal edges of the valves 

 are the projecting hinge teeth that interlock with one an- 

 other. On each valve, toward the anterior end, is a promi- 

 nent elevation, the umbo, which is the oldest portion of the 

 shell. Additions, marked by concentric rings, have been 



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