GASTROPODS. 



35 1 



up like a rope, in the same plane. They frequent stagnant pools and ditches, or 

 slowly running water, in all parts of the world. The well-known P. corneus is 

 not only by far the largest of the eleven British species, but also larger than any 



common pond-snail (Limrwea). 



other known form, although some of the South American types approximate 

 closely. When disturbed, it emits a purple-coloured fluid, probably as a means 

 of defence. 



In Physa the animal is always sinistral, having the respiratory and genital 

 orifices on the left side. The tentacles are cylindrical, and the eyes are at their 



VAEIOUS FORMS OF THE GENUS LIMN^A. 



1, Limncea glabra ;" 2, L. palustris ; 3, L. stagnalis ; 4, L. truncatula ; 5, L. peregra ; 6, L. vulgaris ; 



7, L. ovata ; 8, L. auricularia. 



inner base, as in Limncea. The mantle is furnished on each side with more or less 

 elongate lobes, which, when the mollusc is crawling, are folded back upon the 

 exterior of the shell. In their habits these snails resemble the preceding genus, and 

 they are almost cosmopolitan in their distribution. Physopsis, a Central and South 



