CHAPTER XXI 



FRESH-WATER MUSSEL AND THE SNAIL, OF 



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 rasp- 

 ing food. 



Approximately 78,000 species of moUusks 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 diver- 

 sity of habitats, occurring abundantly on land, in fresh water, and 

 in the sea. Although most of the species live in moist surroundings, 

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

 strictly carnivorous; many of the snails are herbivorous, 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 

 annelids is a matter for conjecture since some morphologists regard 



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