118 



INTRODUCTION TO ZOOLOGY 



CHAP. 



?h. 



Physa fontinalis (the Fountain Bladder Snail). A very 

 common form on water-cress beds ; rather smaller than the 

 Moss Bladder Snail. Shell J to J an inch 

 long; 4 or 5 whorls. Body whorl rela- 

 tively very large, and spire very short (sec 

 Fig. 68, P.f.). The body is characterised 

 especially by the finger-like processes of 

 the mantle which are reflexed over the 

 shell. 



Genus Hooked Snails. These forms 



Ancylus. ar e sometimes known as Fresh- 



water Limpets, for they cling closely to FIG. GS.P.h., Shell 



the weed or stone on which they are living, of Physa hypnonnn ; 



J i i 5 P>J-> shell of Physa 

 and are therefore liable to be overlooked, fontinalis. 



The shell is thin, brittle, and hood-shaped, 

 with an incomplete spire. They move slowly, and rarely 

 come to the surface to breathe. When moving, very little 

 of the body shows beyond the shell. 



Ancylus fluviatilis (the River Limpet). Shell 

 J to J inch ; spire curved to the right, or 

 centrally placed over the body whorl ; respira- 



. iiir^Ti ii' 



tory opening on the left, .bound only in streams, 

 often on the aquatic " willow moss." 



Ancylus lacustris (the Lake Limpet). Shell 

 - Spire twisted to the left, respiratory opening 



r IG. 69. bhell 



of Ancylus 

 fiuviatilis. 



~i 



i 



on the right. 



Pond Snails which have Gills. 



These snails breathe below the surface of the water by 

 means of a plume -like gill \vhich lies within the mantle 



FIG. 70. One Row of Teetli from the Radula of Pulvdina. 



cavity attached to the mantle. They all possess an oper- 

 culum, a horny plate carried above the foot, which, when 



