294 THE RIVER-SIDE NATURALIST. 



Of the Fresh- Water Snails, there are five genera which 

 inhabit our fresh-waters, and are excellent scavengers, 

 feeding on confervoids and decaying vegetable matter ; 

 they are hermaphrodite, depositing their eggs on stones 

 and aquatic plants, enveloped in masses of a slimy sub- 

 stance. They are also of special value as food for trout. 



The genus Planorbis (flat coil) is at once distinguished by 

 the shell being flat and concave, resembling an ammonite. 

 The animal emits a kind of purple blood, and most of its 

 vital organs lie on the left side of the body. 



There are nine British species, some of them very 

 small, all inhabiting ponds, rivers, lakes, marshes, and wet 

 ditches ; those commonly met with are 



Planorbis lineatus (Fig. 3), often found on duck-weed. 



Planorbis marginatus (Fig. i), often found feeding on de- 

 cayed leaves of the yellow iris (Iris pseudo acorns'), and 

 other water-plants. Planorbis carinatus. (Fig. 2). 



The next two genera are distinguished from Planorbis by 

 having spiral conical shells, the genus Physa having the shell 

 conic oval, or oblong spiral sinistral, i.e., turning from right 

 to left ; the shells having a remarkable polished appearance. 



These little molluscs are mostly found in clear shallow 

 running water. 



There are two British species. The most common is 

 Physa fontinalis (Fig. 6) often found attached to water- 

 cresses and other aquatic plants in running streams, and also 

 in canals and ponds ; some of the shells are milk-white. 



