HYDROIDS. 101 
ents for their calcareous secretions, if their food fails of giving 
the necessary amount; and, by means of the powers of animal 
life, bones, shells, and corals alike are formed. 
The origin of the lime in solution throughout the ocean is 
an inquiry foreign to our present subject. It is sufficient here 
to show that this lime, whatever its source, is adequate to ex- 
plain all the results under consideration. 
I. HYDROIDS. 
The annexed sketch represents a Hydra as it often occurs 
attached to the under surface of a floating leaf—that of a spe- 
cies of Lemna. The animal is seldom over half an inch 
























long. It has the form of a polyp, with long slender tentacles ; 
and, besides these tentacles with their lasso-cells, it has no spe- 
cial organs except a mouth and a tubular stomach. Like the 
fabled Hydra, if its head be cut off another will grow out; and 
any fragment will, in the course of a short time, become a per- 
fect Hydra, supplying head, or tail, or whatever is wanting: 
and hence the name given to the genus by Linnzus. 
