CHAPTER VIII 
MOLLUSKS 
v4, 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 threé 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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