THE POND SNAIL. 



155 



observation ; and every student should keep a number 

 of them awhile for his own study. 



In a fully extended specimen, observe, first, the three 

 distinctive and obvious characters of mollusks: (1) the 

 foot beneath the body, (2) the mantle covering its back, 

 and (3) the shell secreted by the mantle. The foot is the 

 broad, flat disk on which the ani- 

 mal creeps ; the mantle edge is just 

 visible within the edge of the shell. 



The Shell. Study the shell. 

 Observe that it is in one piece 

 (univalve), whereas the shell of the 

 mussel is in two pieces (bivalve). 

 Its parts are named as follows : 



1. The apex is the pointed end. 



2. The aperture is the opening 

 at the large end. 



3. The lip is the outer edge of 

 the aperture. 



4. The lines of growth are par- 

 allel to the lip. 



5. The suture is the spiral groove 

 on the outside. 



DIAGRAM OF POND SNAIL : 

 a, apex; s, suture; sp, 

 spire; I, lip; 1, 2, 3, 4, 

 whorls of shell, the larger 

 ones showing lines of 

 growth (the aperture is 

 filled with the body of 

 the animal) ; c, collar 

 (edge of mantle) pro- 

 truding beneath the lip; 

 h, head;/, foot; e, eye; 

 t, tentacle ; 6, breathing 

 aperture. 



6. The spire comprises all the whorls or turns of the 

 shell taken together. 



7. The columella is the axis of the spire. 



If, when holding the shell with the apex toward you, 

 the whorls, as they proceed from you, turn to the right, 

 above the axis of the spire, the spire is right hand, or 

 dextral ; if they turn the other way, it is left hand, 

 or sinistral. 



In the river snails an operculum will be found, a lid- 

 like piece which completely closes the aperture when 

 the animal is retracted within the shell. 



