144 MOLLUSKS 



THE SNAIL 

 Limnaea or Physa 



General Suggestions. — These animals may be found in 

 almost any lake or stream, and can therefore generally be ob- 

 tained fresh for study. They may be kept alive in water in 

 the laboratory for an indefinite period by keeping the water 

 fresh and providing for them cabbage leaves or other 

 plants suited to their taste. They may be killed extended 

 in warm water, and put in fifty per cent, to seventy per cent, 

 alcohol, or two per cent, to three per cent, formalin for 

 preservation. 



The External Form. — (i) Study the living snail in 

 a glass jar. Note its extended body and the relation it 

 bears to the shell. Has it a distinct head? Do you find 

 eyesf Does the animal have vision? What seems to be 

 the use of the tentacles, or feelers? Observe the fleshy 

 foot. How does it use this for movement? Study the 

 mouth? Can you determine how it eats? Does it bite the 

 leaf on which it feeds? Does this animal have a mantle? 

 Where? Can you tell how it breathes? Do you find a 

 breathing pore? On a living animal note the method of 

 disappearing within the shell. Is the disappearance com- 

 plete enough to be of any advantage ? 



(2) The Shell. Is the shell in one piece, univalve, or 

 in two pieces, as the mussel? Find the apex (the pointed 

 end); the. aperture (the opening); and the lip, or outer 

 edge of the aperture. Do you find lines of growth? Begin 

 with the lip and trace the lines of growth around as in the 

 bivalve? How does this shell differ from one valve of the 



