CHAPTER XI 

 THE MOLLUSCA 



The Molluscs include such animals as clams, snails 

 slugs, devil-fish and their allies. One of the most widely 

 distributed and generally available of the molluscs is the 

 fresh-water clam of which there are numerous species in 

 the lakes, ponds and streams of North America. We shall 

 therefore use it as an introductory type. The two valves 

 of the shell by which the body of the clam is enclosed are 

 secreted by a fold of the body wall called the mantle which 

 hangs down on either side of the body. The shefl grows 

 in thickness by additions from the mantle to its inner sur- 

 face, and in area by additions to the edge, the concentric 

 lines visible on the outside of the shell indicating periods 

 of growth. Where the valves are joined together there is 

 a thick, elastic body called the hinge ligament which acts 

 as a spring to open the shell. The shell is closed by two 

 muscles called adductors which run from one valve to the 

 other. The inner surface of the empty shell shows the 

 marks made by the insertion of the two muscles near either 

 end, and also the line of attachment of the mantle to the 

 shell. 



By removing one valve of the shell one may see the gills, 

 two pairs of which hang in the mantle cavity, a pair on 

 either side of the body. These gills are made of numerous 

 fine filaments joined together so as to form broad plates 

 or lamella, hence the term lamellibranch which is applied to 

 the group to which the clam and other molluscs with bi- 

 valved shells belong. Each gill is composed of two lamel- 



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