HYDRA. Ill 
CHAPTER XIII. 
TYPES OF C@LENTERATA. 
HYDRA. OBELIA. ACTINIA, ALCYONIUM. AURELIA. CYDIPPE. 
I.—HYDRA. 
PHYLUM - 
Crass 
Fig. 45.—Hypra VIRIDIS 
wirH Two Bups 
(Magnified). 
CaiLENTERATA. 
Hyprozoa. 
Hydra viridis is a small fresh- 
water organism, which may attain 
a length of one-half inch, but is 
usually smaller. It is found in 
ponds and streams attached to 
water-weeds and is of a bright 
green colour. 
[Hydra fusca is the brown 
species ; with the exception of the 
absence of green chromatophores 
it resembles the above. | 
Hydra is axo-symmetric, hence 
we can distinguish merely an oral 
and an aboral end and axial and 
peripheral parts. With the naked 
eye it can be seen that 
the body is an elon- 
gated cylinder fixed at 
the aboral end. At the oral end 
there is a ring of tentacles, thin 
processes which radiate in all 
directions. In the centre of this 
ring is a small raised part, the 
peristome, wpon which is situated 
the mouth. 
We may notice at once that this 
aperture, though usually termed 
the mouth, functions both as a 
mouth and an anus. 
External 
Characters. 
