6*io 
ZOOPHYTES. 
generations continue their operations on the upper 
and lateral surfaces, each according to the particular 
form which Nature has prescribed. The great 
masses of coral rock, according to Mr. Barrow, ap- 
pear to be composed chiefly of madrepores , cellipo- 
res, and tubipores. In order to determine whether 
the central parts of a coral island were composed of 
the same materials as the shore, he removed the 
soil to the depth of about three feet, and found 
large blocks of madrepores, with various cellular 
masses of calcareous formation. 
Thus by imperceptible gradations, and in an al- 
most inconceivable manner, do these apparently in- 
significant beings raise huge masses of durable rock, 
which at length appear above the surface of the 
sea, and soon become covered with vegetables. 
The stony habitations of these little creatures, in 
which they can conceal themselves at pleasure, are 
highly necessary for their preservation, and defend 
them from the rude attacks of a boisterous element, 
as well as from the many enemies which constantly 
surround them. The fresh-water polype, on the 
contrary, which we are now about to describe, being 
confined to the calm and sheltered situation of ponds 
and ditches, does not stand in need of so necessary a 
provision for its defence, and is therefore denied 
the power of forming any covering. 
