CHAPTER XIX 



ALLIES OF THE ACEPHALA: MOLLUSCA 



The frugal snail, with forecast of repose, 

 Carries his house with him where'er he goes. 



Charles Lamb 



The Pond Snail. Any one of several genera and species 

 of snails that live in fresh water might be given the name 

 "pond snail" with equal accuracy. The one represented in 

 Fig. Ill (Phy'sa) is quite common not only in ponds but in 

 rivers as well. 



Externally the most noticeable feature of the pond snail is 

 its thin, spiral shell. This structure is made of the same ma- 

 terial as the shell of bivalves. A large portion of the animal's 

 body fills the "mouth" of the shell and extends spirally 

 toward the top. Between the shell and the body wall lies 

 the mantle, which throughout life continues the growth of 

 the shell in the characteristic spiral direction. 



When the snail is disturbed it draws the entire body into 

 the shell ; but when it is feeding, as shown in the picture, 

 one can see the long, muscular foot, broad in front and 

 pointed behind. The anterior region of the body is the head, 

 more or less sharply marked off from what lies behind. The 

 mouth opens beneath an extensile upper lip. In the mouth 

 is a short, muscular tongue, on which grows a long but 

 minute ribbon of rasp-like teeth. The pond snail uses this 

 rasping tongue to tear into bits the soft plant tissues on 

 which it feeds. A most interesting experience is to watch 

 a pond snail as it crawls slowly up the side of an aqua- 

 rium, feeding on the microscopic plants as it goes. At 



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