CHAPTER III. 
BRANCH III.—CQiLENTERATA (Hypro1ps, JELLY- 
FisHEs AND PoLyps). 
General Characters of Colenterates.—In this branch, 
which is represented by animals like the Hydra (Fig. 36) and 
Tubularia (Fig. 35), the body consists 
of two cell-layers, surrounding a 
definite, single, digestive cavity, the 
mouth of the cavity being surrounded craSs 
by a circle of tentacles, which are in 
polyps hollow and connect with the 
stomach. The latter, however, is only 
partly differentiated or set apart from 
the body, hence the name Celenterata 
(Greek, o1A0s, hollow, and évrepor, 
digestive tract). From the stomach 
often radiate water-vascular canals, no 
blood-system yet appearing thus far in 
the animal kingdom, the products of 
digestion reaching the tissues from 
the smaller branches of the primary 
water-vascular canals. The nervous 
system is either absent, or in different 
grades of development, from the iso- 
lated nervo-muscular cells of Hydra SALTS 
and the scattered nerve-cells of an rg, 35.—-A Hydrotd, Tube 
Actinia, to the continuous ganglion- {27i2.,,% meduen buds s ef, 
ated nervous ring of the minute From Tenney's Zoology. 
jelly-fish such as Sarsia. These aniraals display a striking 
amount of radial symmetry, the organs and body being dis- 
posed in a radiate manner around a central vertical axis, in 
