312 



Sliell Life 



carbonic acid of the pond to attack the lime of 

 the shell material. It is a general feeder, but with 

 a distinct bias in favour of animal matter. It will 

 even attack and kill newts and sticklebacks, whose size 

 and agility might be considered more than sufficient 

 to save them from such a fate. It also destroys 

 the larva3 of water-beetles in the same way; but 

 the mature Dythscibs retaliates by eating stagvaUs. 

 It must be confessed, too, that the Great Pond- 

 snail is, at least 

 occasionally, a 

 cannibal, de- 

 stroying young 

 individuals of 

 its own kind. 

 : Its movements 

 are graceful, 

 whether it be 

 ascend in o' or dc- 

 I scending aquatic 

 vegetation or 

 gliding inverted 

 along the sur- 

 face of the water. 

 They freciuently come to the surface to discharge the 

 effete air in the lung-chamber and take in a fresh 

 suppl}^ Full-sized specimens of the Great Pond-snail 

 need be sought only in large ponds ; and it appears to 

 be a well-established fact that the rate of development 

 and the ultimate size attained are in direct propor- 

 tion to the volume of water in which the individuals 

 have lived. Karl Semper set this point beyond 

 dispute by separating the fry from a single batch 



Great Pond sn; 



