I 82 



Aquatic Organisms 



they feed, their work is little noticed; yet they con- 

 sume vast quantities of green tissue and dead stems. 

 The commoner pond snails lay their eggs in oblong 

 gelatinous clumps that are outspread upon the surfaces 

 of leaves and other solid supports. Other snails are 

 viviparous. 



The two principal groups of fresh -water snails may 

 roughly be distinguished as (i) operculate snails which 

 live mainly upon the bottom in larger bodies of water, 

 and have an operculum closing the aperture of their 

 shell when they retreat inside, and which breathe by 



Fig. 89. Two pond snails {LimncBa palustris) foraging 

 on a dead stem that is covered with a fine growth of 

 the alga, Chcetophora incrassata. 



means of gills: (2) pulmonate snails, which most 

 abound in vegetation-filled shoals, breathe by means of 

 a simple lung (and come to the surface betimes, to refill 

 it with air) and have no operculum. 



The snails we oftenest see are members of three 

 genera of the latter group: LimncEa, shown in the 

 accompanying figure, having a shell with a right-hand 

 spiral and a slender point; Physa, having a shorter 

 spiral, twisted in the opposite way, and Plajiorbis, 

 shown in fig. 65 on p. 155, having a shell coiled in a flat 

 spiral. A ncylus is a related minute limpet-shaped snail, 

 having a widely open shell that is not coiled in a spiral. 

 Its flaring edges attach it closely to the smooth surfaces 

 of plant stems or of stones. 



