118 



INTRODUCTION TO ZOOLOGY 



Ph. 



^^^# 



Physa fontinalis (the Fountain Bladder Snail). — A very 

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

 Moss Bladder Snail. Shell ;J to | an incli 

 long ; 4 or 5 whorls. Body whorl rela- 

 tively very large, and spire very short (sec 

 Fig. 68, P./.). 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. are sometimes known as Fresh- 

 water Limpets, for they cling closely to Fib. 68.— p.h., Shell 

 the weed or stone on which they are living, pf^lZnofTh^m 

 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 

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

 centrally placed over the body whorl ; respira- 

 tory opening on the left. Found only in streams, 

 often on the aquatic " willow moss." 



Ancylus lacustris {the Lake Limpet). — Shell 

 to ^ inch. Spire twisted to the left, respiratory opening 

 on the right. 



Fig. 69.— Shell 

 of Ancylus 



10 



Pond Snails which have Gills. 



These snails breathe below the surface of the water by 

 means of a plume -like gill which lies within the mantle 



Fig. 70. — One Row of Teeth from the Radula of Paludina. 



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

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



