180 



GENERAL ZOOLOGY 



surface quickly. Conversely, if the animal voluntarily (or 

 under visible stimulus) 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 instance the 

 weight of the snail's body 

 is greater than the weight 

 of an equal volume of 

 water. One might well 

 ask why the pond-snail 

 doesn't migrate up and 

 down in this way habit- 

 ually. We can only say 

 that perhaps the sudden 

 change of pressure from 

 a great to a less amount 

 in going up, and from a 

 small to a greater amount 

 in going down, is not so 

 favorable as a slower rate 

 FIG. 02. Mucus-Threads of Pond-Snail o f change would be. 

 (Limnoza) in Water. Reduced. (After 



Kew) We find some confir- 



mation of this theory in 



the existence of vertical threads of mucus in snail aquaria 

 and in ponds (Fig. 92). A snail on leaving the bottom 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 upward, slowly 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 sexual gland is hermaphroditic, hence 



