120 



INTRODUCTION TO ZOOLOGY 



the glass of the tank if this is tapped whilst the snail is 

 climbing up it. In the male the right tentacle is shorter and 

 thicker than the left. 



Genus Bithynia consists of much smaller forms which 



Bithynia.i are oviparous ; the eyes are sessile ; the operculum 



is calcareous and therefore hard and brittle ; the tentacles are 



thread-like and are both the same size in the male. The 



eggs are laid in two rows in a gelatinous ribbon, each egg 



Fig. 72. — Bithynia tentaculata. 

 A, Snail extended, showing operculum ; B, eggs ; C, closed shell seen from below. 



being surrounded by a definite hexagonal area of jelly (see 

 Fig. 72). 



Bithynia tentaculata. — Shell ^ inch long and with 6 whorls ; 

 aperture of shell oval. This is rare in the north of England. 

 Bithynia Leachii. — Shell ^ inch long and with only 

 4 whoris, also the whorls are more 

 rounded ; aperture of shell nearly 

 round. This form is only found 

 in the south of England. 



Marine Univalve Molluscs. 



Fig. 73.— The Common 

 Periwinkle. 



Genus 



Littorina. 



Periwinkles. 



One species of this 

 genus is Littorina 

 Uttwalis, the common 

 Periwinkle, very similar in structure and habits to Paludina, 



' The name means "inhabiting deep water," but it is rather misleading, 

 as this snail lives in poncls with Limnaea and Planorhis. 



