FIELD BOOK OF PONDS AND STREAMS 



snail can take enough air into its lung to last it for a long time 

 under water. An occasional bubble can often be seen escap- 

 ing from lung-sacs of snails thus submerged. 



In some snails the male and female sex organs are in dif- 

 ferent individuals ; others are hermaphrodites with complica- 

 ted male and female systems within the same animal. Usually 

 two animals mate together, whether hermaphrodites or not. 

 In the family Lymnmidm (Fig. 245) there are some evidences 

 of self fertilization. Alany snails lay eggs, depositing them 

 in masses of protecting jelly (Fig. 244), others, such as Cam- 

 peloma, are viviparous, bearing their young alive. 



Fig. 244. — Cluster of eggs of pond snail, Physa, 

 showing the eggs within their capsules of jelly and 

 these embedded within the general mass. 



Eggs. — Snails lay their eggs on any submerged objects, espec- 

 ially upon plants and protected regions of stones. The little 

 groups of them are embedded in drops of crystalline jelly. 

 There are never many eggs in a clutch, each one is surrounded 

 by its individual capsule of jelly and all the capsules are sur- 

 rounded by the general mass (Fig. 244). Developing snails 

 can be clearly seen through all the jelly; they turn over and 

 over within their capsules, and as they grow older their black 

 eyes and snail hum.p are clearly recognizable through a hand- 

 lens. 



Pond snails, Family Lymnaeidae. — These are air-breathing 

 snails whose shells are thin, with a right-handed coil (Fig. 

 240), an acute spire and a large body aperture. The animal 

 has a short, rounded foot and flattened, triangular tentacles. 

 These snails live in lakes, ponds, swamps, and smaller streams 



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