CHAPTEK VIII 



MOLLUSKS 



74. General characters. For very many years the mol- 

 lusks that is, the clams, snails, cuttlefishes, and their allies 

 have been favorite objects of study largely because of the 

 durability, grace, and coloration of the shell. The latter 

 may be univalve, consisting of one piece, as in the snails, or 

 bivalve, as in the clams and mussels, and may possess almost 

 every conceivable shape, and vary in size from a grain of 

 rice to those of the giant clam (Tridacna) of the East Indian 

 seas, which sometimes weighs five hundred pounds. These 

 external differences are but the expression of many internal 

 modifications, which, while adapting these animals for dif- 

 ferent modes of life, are yet not sufficient to disguise a 

 more fundamental resemblance which exists throughout 

 the group. In some respects the mollusks show a close 

 resemblance to the annelid worms, but, on the other hand, 

 the body is usually more thick-set and totally devoid of any 

 signs of segmentation. In every case the skin is soft and 

 slimy, demanding moist haunts and usually the protection 

 of a shell, and the body is modified along one surface to 

 form a foot or creeping disk which serves in locomotion. 

 The internal organization is somewhat uniform, and will 

 admit of a general description later on. Mollusks are 

 divided into three classes, viz. : The Lamellibranchs, em- 

 bracing the clams ; the Gasteropods, or snails ; and the 

 Cephalopods, or cuttlefishes, squids, and related forms. 



75. Lamellibranchs (clams and mussels). Numerous rep- 

 resentatives of this class, such as the clams and mussels, 



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