MOLLUSC A 



119 



the animal withdraws into its shell, just fits into and closes 

 the mouth of the shell. This operculum is formed as a 

 cuticular structure by the secretion of a group of cells on 

 the dorsal side of the foot, at its posterior end. 



Genus Fresli-water Winkles. — These snails are vivi- 



Faludiua. parous, i.e. the eggs are hatched within the body 

 of the mother ; the fry are not set free until they are at least 

 two months old, when they are gradually ejected, two or 

 three at a time. The two British 

 species of this genus are both large 

 snails having a shell of 6 or 7 

 whorls with very convex surfaces ; 

 in both of them the foot bears on 

 its upper surface a horny plate, or 

 "operculum," with irregularly con- 

 centric lines of growth on it. When 

 the snail withdraws into its shell, 

 this plate completely closes the 

 aperture. There are two tentacles, 

 with an eye placed on a little 

 tubercle on the outer side of the 

 base of each. The mouth is borne 

 on a prominent snout. 



Paludina vivipara (see Fig. 71). — 

 This species is fairly common in 

 ponds and slow- flowing streams. 

 The body is dark, speckled with 

 yellow. The shell has a rather 

 thick operculum, and there are 

 three dark bands on the body whorl 

 of the shell, running spirally round 

 it. The form of the teeth on the 

 rasp is shown in Fig. 70. 



Paludina conteda. — This species 

 is not so common as vivipara, though 

 not rare in the south of England. 

 It is slightly larger than the other 

 species and the shell has a thinner 

 operculum, more swollen whorls, a 

 more pointed spire, and a more distinct umbilicus. It is a 

 very irritable creature, falling with surprising rapidity from 



6 b ^ 



Fig. 7 1 . — Paludina vivipara. 



A, Female snail ; B and C, males ; 

 /, foot ; &, breathing aperture ; 

 0, operculum. 



