CQ@LENTERATA. 133 
Cydippe resembles Actinia in the presence of an ectodermal gullet 
and of central and peripheral portions of the ccelenteron, but it differs 
from all the preceding types in the possession of ‘‘ combs” of cilia. 
PHYLUM CCELENTERATA. 
The Phylum Ccelenterata is extensive and of great 
zoological importance. The six types described above 
(Z.e., Hydra, Obelia, Aurelia, Actinia, Alcyonium, Cydippe) 
give a good general idea of its organisation and place in 
nature, 
They are mostly marine, all aquatic and all retain the 
primary metazoan axis, about which they are usually 
axo-symmetric though, as in the last two types, they may 
progress to bi-plano-symmetry. They are usually either 
sedentary or pelagic. R 
In structure they are all formed of two epithelia (or 
derms), an outer layer or ectoderm and an inner or endo- 
derm, between which is a thin or thick mesoglea. This 
two-layered condition has-been compared to that of the 
typical diploblastic larva, the gastrula. The comparison is 
as follows :— 
GASTRULA,— Ca.ENTERATA.— 
Epiklast. Ectoderm. 
Hypoblast. Endoderm. 
Archenteron. Ccelenteron. 
Blastopore. Mouth. © 
Central axis. Primary axis. 
We can divide CeZenferata into three classes :— 
1. Hydrozoa. 
2. Scyphozoa. 
3. Ctenophora. 
Crass I.— Hyprozoa. (Hydra and Odelta.) 
In these animals the ccelenteron remains simple, the 
axial symmetry is undisturbed and there is no ectodermal 
gullet. They include hydra-like forms with only a hydroid 
phase ; obelia-like zoophytes which have a hydroid and a 
medusoid phase (though the medusoid may be degenerate) ; 
and others (¢.g., Va/comedusa@) with only a medusoid phase. 
