166 



ZOOLOGY 



at high tides. One of the commonest forms is Melampus 

 (Fig. 152), found among the roots of marsh grass. 



Of the aquatic pulmonates three genera are common and 

 easily distinguishable. Limnaea, 1 the '" pond snail," is com- 

 mon in ditches and muddy or stagnant ponds. Its shell 

 runs up into a sharp spire, and is right-handed, i.e. hold- 

 ing the shell so that its aperture 

 is next the observer and below, 

 the aperture is at the right 

 (Fig. 153). Limnsea crawls over 

 the bottom, up the stalks of 

 aquatic plants, and on the sur- 



FIG. 153. -Left, Physa hetcro- faCG film f Water ' During 



stropha, the left-handed pond drought it burrows into the 

 snail. Right, Limnaea. the right- i i i ,-, P 



handed pond snail, with the lliud > aild cl SeS the aperture of 



apex eroded off as is usually the the shell . 

 case in adult shells. Nat. size. -ri_ o i 



Photo.byW.H.c.P. Pnysa^ has a smaller, rela- 



tively stouter shell than Lim- 

 nsea, and one whose coil is left-handed (Fig. 153). It 

 lives in even the smaller ponds and brooks, and may be 

 easily reared in aquaria. It feeds freely 

 upon any kind of vegetable matter. 

 Physa heterostropha is the common species 

 of the United States (Fig. 153, left). 



Planorbis 3 is coiled in one plane like a 

 watch spring. 4 It lives in a similar 

 habitat with Physa. A great many 

 species are distinguished which vary con- 

 siderably in size. Snails of this genus 

 likewise are easily kept in the aquarium, and lay numerous 



FIG. 154. Planorbis, 

 the flat-coiled pond 

 snail. Nat. size. 

 Photo.byW.H.c.P. 



1 From \Lfjivij, a marsh. 



2 <0<ra, bellows, 



3 planus, flat ; orbis, circle, 



4 Fig. 154, 



