Gastropoda, 



133 



The Pond Snails. The pond snails have the mantle 

 cavity transformed into a lung, as in the land snails. 

 They frequently come to the surface of the water to get 

 air, and may be seen first to emit a bubble of the contained 

 air, take a new supply, and again descend to resume their 



FIG. 85. FIG. 86. 



POND SNAILS CREEPING. 



The largest part extending from the shell is the foot. There are three other protruding 

 organs: (i) the proboscis, in the center; (2) the two tentacles, with an eye at the base 

 of each; (3) outside the tentacles, theTespiratory tubes, one of which takes in water, the 

 other sending it out. In Fig. 85, the dark semicircle back of the shell is the operculum. 



eating. They are exclusively herbivorous, and in an aqua- 

 rium may be observed cleaning off the layer of green scum, 

 mostly consisting of algae, which grows on the sides of the 

 aquarium. 



FIG. 87. VARIATIONS IN A COMMON POND SNAIL. 



After Morse, from Packard's Zoology. 



There is but one pair of tentacles, at the bases of which 

 are the eyes. Only a few pond snails have operculums. 



