CHAPTER XVII 



PHYLUM MOLLUSCA 

 (By Elmer P. Cheatum, Southern Methodist University) 



GENERAL CHARACTERS 



The phylum Mollusca includes such familiar animals as the snails, 

 clams, oysters, and cuttlefish. Even though they appear different 

 externally, all are soft-bodied, unsegmented, usually bilaterally 

 symmetrical, and most of them produce a shell composed princi- 

 pally^ of calcium carbonate. A muscular foot is present which may 

 be modified for different functions. In the snail it is used for 

 creeping ; in the clam for plowing through the substrate, and, in the 

 nautilus or squid for seizing and holding prey. Covering at least a 

 portion of the body is a mantle or dermal fold, the outer surface of 

 which secretes the shell in most species. Between the mantle and 

 main body is a mantle cavity which is usually either provided with 

 gills or modified into a primitive pulmonary sac for use in respira- 

 tion. Jaws are present in the snails, slugs and cephalopods. Within 

 the mouth cavity of many species is the radula, which is an organ 

 composed of fine chitinous teeth arranged in rows and used in 

 rasping food. 



Approximately 78,000 species of mollusks have been described, 

 hence they constitute one of the largest groups of animal life. 

 With very few exceptions they are sluggish animals and occupy a 

 diversity of habitats, occurring abundantly on land, in fresh water, 

 ajid in the sea. Although most of the species live in moist sur- 

 roundings, a few inhabit arid regions. Some species, such as the 

 cuttlefish, are strictly carnivorous; many of the snails are herbivo- 

 rous, and others feed as scavengers. The oyster and other species 

 that are attached during adulthood feed on the floating organisms 

 in the sea. 



From the standpoint of their ancestry, the veliger larva of vari- 

 ous marine forms bears close resemblances to the trochophore larva 

 of the annelids. Whether or not they are direct descendants of the 



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