CHAPTER XV 

 ALLIES OF THE PELECYPODA : 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. 91 (Phy'sa heterostro'pha) 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 material 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. The direction of the spiral (or spire) 

 in Physa is left-handed, an unusual condition. The spire 

 of a left-handed shell, starting with the top toward the ob- 

 server, turns contrary to the movement of the hands of a 

 clock ; the spire of a right-handed shell turns with the hands 

 of a clock. 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 its shell down over 

 the entire body; but while 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 



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