Corals and coral islands.

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 explain all the results under consideration.
II. HYDROIDS.
The annexed sketch represents a Hydra as it often occurs
attached to the under surface of a floating leaf-that of a species of Lemna. The animal is seldom over half an inch
- A
HYDRA.
long. It has the form of a polyp, with long slender tentacles;
and, besides these tentacles with their lasso-cells, it has no special 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 perfect Hydra, supplying head, or tail, or whatever is wanting:
and hence the name given to the genus by Linnaeus.