BRANCH MOLLUS'CA 



These animals have soft, unsegmented bodies, as contrasted 

 with the segmented Arthropoda. The body is generally bi- 

 laterally symmetric, but it may be asymmetric, as in the snail. 

 They vary in size from a fraction of an inch to from 2 to 5 feet 

 in length; and in weight from a fraction of an ounce to 500 

 pounds. The body may be naked, as the slug; or covered 



Fig. 52. — Part of a bunch of oysters from Great Point Clear Reef, showing 

 attachment of barnacles and mussels. (Bulletin, U. S. F. C, 1895.) 



with a univalve shell, as the snail; or with a bivalve shell, 

 as in the common mussel; or it ma>' have an internal horny 

 pen, as in the squid. The structure and form of the INIoUusca 

 are very various, and the number of known living and fossil 

 species exceeds forty thousand. Some mollusks are marine, 

 some are fresh-water forms, and others are terrestrial. 



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