CCELENTERATA. I33 



Cydippe resembles Actinia in tlie presence of an ectodermal gullet 

 and of central and peripheral portions of the coelenteron, but it differs 

 from all the preceding types in the possession of ' ' combs " of cilia. 



PHYLUM CCELENTERATA. 



The Phylum Coelenterata is extensive and of great 

 zoological importance. The six types described above 

 (/.(?., 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 bf-plano-symmetry. They are usually either 

 sedentary or pelagic. 



In structure they are all formed of two epiihelia (or 

 derms), an outer layer or ectoderm and an inner or endo- 

 derm, between which is a thin or thick mesogloea. This 

 two-layered condition has been compared to that of the 

 typical diploblasiic larva, the gastrula. The comparison is 

 as follows : — 



Gastrula. — Ccei.enterata. — 

 Epiblast. Ectoderm. 



Hypoblast. Endederm. 



Archenteron. Coelenteron. 



Blastopore. Moulh. 



Central axis. Primary axis. 



We can divide Ccelenterata into three classes : — 



1. Ilydrozoa. 



2. Scyphozoa. 



3. Ctenophora. 



Class I. — Hydrozoa. (Hydra and Obelia.) 



In these animals the coelenteron 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 [^e.g.^ Naicomedusce) with only a medusoid phase. 



