FRESH-WATER SNAILS. 7 



6. Tlie axis around which the whorls revolve is called 

 the columella. This axis is generally solid, though in many 

 shells it is hollow, as if the whorls had turned around a 

 shaft which had afterward been withdrawn. This hollow 

 axis looks like an opening in the base of the shell, as in 

 the following figure : 



FIG. 10. A SNAIL-SHELL SEEN FROM BELOW. 



This opening is called the umbilicus. The apex of the 

 shell is sometimes called the nucleus, because the shell com- 

 mences to grow from this point. 



CHAPTER II. 



FRESH-WATER SNAILS. 



7. THE pupils will now be required to bring in some live 

 snails. Let them examine bits of bark, chips, or branches, 

 found in ditches, or muddy brooks. Under lily-pads and 

 on the stems and leaves of other aquatic plants, and on 

 stones in rivers, snails of various kinds will be found. A 

 dipper with the bottom perforated, or made into a sieve, and 

 attached to a wooden handle four or five feet in length, will 

 be found useful in scooping up the sand or mud from the 



