MOLLUSC A 



117 



Planorbis contortus (the Twisted Trumpet Snail). Shell i 

 inch in diameter. Whorls 8, very similar to vortex, but about 

 half the size and more common ; 

 rather a sluggish snail. The shell 

 has a crescent-shaped opening, is 

 much compressed, and has no keel ; 

 the left side is deeply concave. 

 Widely distributed, but rather local. 



Planoi'bis spirorbis (the " Round- 

 spired" Trumpet Snail). Shell J 

 inch in diameter. Whorls 5 or 6. 

 One side of shell concave and one 

 flat or concave, keel rather blunt. 

 Common in stagnant shallow water 

 or grass. 



Planorbis carinatus (the Keeled 

 Trumpet Snail). Shell \ inch in 

 diameter, with 5 or 6 whorls only ; 

 a distinct keel running down the 

 centre of the whorl. 



There are many other species 

 which are not, however, so common, 

 and the characteristics of which 

 have not in many cases been very 

 clearly determined. 



Genus Physa. Ihe ^ M ? 

 are easily recognised 



by the much-polished sinistral shell 

 (see p. 101). They are active little 

 creatures, with long, slender ten- 

 tacles, and with the habit of sup- 

 porting themselves in the water by 

 mucous threads, on account of 

 which they are sometimes called 

 " Thread-spinners." 



Physa ( = Apleda) hypnorum (the 

 Moss Bladder Snail). Shell J to f 

 inch long, 5 or 6 whorls. Glossy 

 yellow or reddish, aperture of shell 

 pear-shaped (see Fig. 68, P.h.). No finger-like processes extend 

 from the mantle over the shell. Of rather local occurrence. 



FIG. 67. Some common 

 Planorbis species. 



com., P. complanatus (slightly 

 enlarged) ; P. vor., P. vortex x 2 ; 

 P. cont., P. contortus x 2 ; P. 

 spir., P. spirorbis x 2 ; P. car., P. 

 carinatus x 2. 



