CHAPTER III. 
THIRD BRANCH OF ANIMALS, 
HYDROIDS, Etc. (Celenteraia, hollow intestine). 
General Characteristics.—A. simple sac, as the Hydra, 
composed of two-cell layers, possessing a stomach, or di- 
gestive cavity. The mouth is encircled by tentacles, which 
are hollow, and connect with the stomach. 
Class I.—Hypbrozoa. 
Order I. Hydroids.—One of the commonest animals 
of the aquarium, if the water is taken from the brook or 
stream, is the Hydra 
(Fig. 13)—a simple, 
elongated stomach 
one quarter of an 
inch in length, end- 
ing in a mouth that 
is surrounded by from 
five to eight tentacles 
that are extensions of 
the body, hollow, and 
connecting with it. 
If examined closely, 
myriads of small cells 
will be seen, many of 
which contain deli- 
Fic. 13.—Fresh-water Hydra viridis. 1. 
Long-armed Hydra feeding on small ani- 
mals, a. 2. Hydra fusca throwing off 
young Hydra-buds. 
cate threads or darts, called lassoes, that are thrown out 
as weapons of defense (Fig. 14). By cutting a Hydra 
