210 



GENERAL ZOOLOGY 



When the foot is extended in locomotion, the pond snail 

 weighs less than an equal volume of water. If the animal 

 releases its hold on an object at the bottom, it floats to the 

 surface quickly. Conversely, if the animal releases its hold 

 on the surface of the water, it draws the entire body into 

 the shell and quickly falls to the bottom. In the second in- 

 stance the weight of the 

 snail's body is greater 

 than the weight of an 

 equal volume of water. 



We find vertical threads 

 of mucus in snail aquaria 

 and in ponds (Fig. 112). 

 A snail on leaving the bot- 

 tom may pour out mucus 

 from its foot-gland in the 

 usual way. The mucus 

 fastened at the bottom 

 will be paid out in the 

 form of a thread as the 

 animal floats slowly up- 

 ward, held back by this 

 thread. When the snail gets to the surface the thread is 

 moored there in a patch of mucus. Each thread thus formed 

 becomes a permanent pathway, tending to increase in thick- 

 ness and in strength as the snail makes use of it. 



In the spring and summer months the pond snail lays 

 eggs even in captivity. The gonad is hermaphroditic, hence 

 every individual is likely to deposit eggs. The eggs may be 

 found on the branches of water plants, or even on the per- 

 pendicular sides of a glass aquarium, imbedded in an ellip- 

 tical, clear, gelatinous mass from two millimeters to six 

 millimeters long. In each mass from five or six to twenty- 

 five eggs may be distinguished. 



Fig. 113. Living land snail. (Slightly 

 reduced) 



