3 26 Shell Life 



which are shown in our ligure. It is a very active 

 little creature, creeping over the water-plants and 

 skimming the surface of the water from beneath. 

 Its home is chieil}^ in sluggish streams, brooks, and 

 ditches. The ]\loss Bladder - snail (P. hypnoru'tii) 

 cannot well be mistaken for its congener, for its 

 longer shell has a very distinct spire, and it 

 is not even partially hidden by mantle-lobes. 

 It appears to delight in basking in the sun as 

 it glides along the surface of its ditch. It is 

 very unreliable in its haunts. I have sought 

 it where a few days previously it had been 

 abundant, but not a specimen was to be found. 

 Such sudden appearances and disappearances 

 can only be explained by supposing the mollusk 

 spends much of its time out of water; but some 

 of the stories told of its sudden appearance in dis- 

 tricts where it liad previously been unknown ai'e 

 in the nature of puzzles. It prefers ditches wliich 

 dry up in the summer. It is worth noting that 

 so far as at present known this is the most 

 northern of all the Pulmonate mollusks, and has 

 been found living on the peninsula of Taim^a- in 

 northern Siberia, where the mean annual tempera- 

 ture is below 10° F. A larger species, P. acuta, 

 appears to have been introduced to Kew Gardens 

 with West Indian plants, and now it is thoroughly 

 acclimatised in the water-lily tank. 



The Bladder-snails have a trick of spinning threads 

 of mucus as they rise to the surface, and by allowing 

 a short length of it to lie on the water so hx it that 

 they can use it repeatedly as a direct way up or 

 down. This mucus thread is not like the byssus of 



