SNAILS AND LIMPETS. 



175 



rather blunt at their extremities. The shell is about gin. in 

 diameter, compressed, concave abqve, and almost flat below. 

 The whorls are five or six in number, and rapidly enlarge. 

 The sutures are deep ; the aperture is angular. 



Planorhis i/narginatus (Fig. 117) is very like P. carinatus in 

 structure and habits ; but it is commoner, though quite as 

 useful in the aquarium as its near relative. Great numbers 

 of P. ma/rginatus are said to be bred in those reservoirs in 

 Yorkshire which are used for cdoling the water from the con- 

 densers of the steam- 

 engines of the factories. 



The tentacles of P. mar- 



ginatus are rather more 



pointed than those of 



P. carinatus. In other 



respects the ' bodies of 



the two animals are 



much alike. The shell 



of P. marginatus is 



thicker, and a little 



greater in diameter than 



that of P. carinatus, and 



the keel is placed nearer 



to the lower side of the 



periphery than to the 



middle; hence the 



specific name marginatus. 



as that of P. carinatus. 



FiQ. 118. Planorbis vortex. 



The keel itself is not so prominent 

 The eggs of this species are from 

 six to fourteen in number, and are contained in an orbicular 

 capsule; they are hatched, according to the temperature of 

 the water, in about a fortnight. 



Planorbis vortex (Fig. 118) is a very pretty little snail 

 foimd in the slow-running and stagnant water of nearly 

 every part of Britain. It is one of the molluscs which are 

 in the habit of closing the aperture of their shell with an 

 epiphragm should the pond, ditch, or stream in which they 

 live become dry during summer; and thus protected, it 

 will remain in a state of torpidity until the return of rain. 



