CCELENTERATA. 
247 
a central cavity with one 
opening. The animal 
may be compared to a 
bag with a two-layered 
wall, and tentacles around 
the opening. It buds, 
and also reproduces by 
eggs. The buds, when 
adult, become detached 
from the parent. 
In most of the other 
Hydroids the colony is 
permanent, and support¬ 
ed by a horny skeleton. 
There are two kinds of 
Polyps in each colony, 
one for feeding and the 
5 
Fig. 191.—Hydra: 2, with tentacles fully extend¬ 
ed ; 3, creeping; 5, budding. 
other for reproduction. 
Fig. 192.—Hydroid ( Hertularia ) growing on a Shell. 
Sometimes the reproductive 
Polyps are separated 
from the stock in the 
form of little Jelly¬ 
fishes. The larger 
Jelly - fishes belong 
to another group 
—the Acalepha— 
and are produced as 
told on page 212. 
The Jelly - fish has 
a soft, gelatinous, 
semi-transparent,bell¬ 
shaped body, with 
tubes radiating from 
the central cavity to 
the circumference, 
and with the margin 
