HEDGE AND DITCH 165 



in an aquarium. In many respects it is like the garden- 

 snail, but it has its points of difference. The tentacles 

 are of a different shape, being flattened at the base, and 

 the eyes, instead of being borne at the tips of the tentacles, 

 are placed at their bases, on the inner side. The tentacles 

 of a pond-snail cannot be telescoped, like those of the 

 garden-snail. The broad creeping foot, the mantle applied 

 to the shell, and the breathing-pore are much the same 

 in both animals. In these, as in all snails, the tip of the 

 spiral shell is turned away from the head, and the mouth 

 of the shell opens on the right side. 



Pond-snails, and some other aquatic mollusks, are fond 

 of creeping on the surface-film of water. Semper 1 and some 

 other naturalists have tried to correct this statement ; 

 they maintain that the snail creeps on the air. A simple 

 experiment will decide who is right. When a pond-snail 

 is found travelling foot uppermost at the top of a tank, 

 dust the water with lycopodium or some other light powder. 

 You will see that as the snail travels along it does not part 

 the thin layer of powder, but glides beneath it. Hold a 

 wet finger over it, and let a big drop fall upon the foot ; 

 this looses the hold upon the surface-film, and causes the 

 animal to capsize. As it is lighter than water in conse- 

 quence of its air-filled lung the snail continues to float ; 

 only when seriously alarmed does it expel a bubble of air 

 from its lung, and sink to the bottom. To regain its 

 position at the surface, when it has sunk to the bottom, 

 the snail must climb up the side again, which might be a 

 long and toilsome business in a large pond. Threads of 

 slime are occasionally used by pond-snails to facilitate 

 ascent or descent, and I believe that if the threads were 

 easily visible, we should find that they are often employed 

 to get over a difficulty. It appears from the testimony 

 of several observers that a pond-snail, when rising from 

 the bottom by virtue of its buoyancy, may check its 

 ascent by a thread attached below ; that when sinking, 



1 Animal Life, p. 295 and note 97 (Eng. Trans.). 



