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CCELENTERATA. 



PHYLUM CCELENTERATA. 



1. Metazoa with radial (axial) symmetry. 



2. Body of two layers of cells, ectoderm and endoderm, enclosing one continuous 



gastric cavity, which communicates to exterior by one opening, the mouth- 

 anus. * 



3. A structureless lamella, the mesogloea, between the two layers. 



4. Ectoderm cells bear nematocysts. 



5. Asexual reproduction by budding produces colonies. 



6. Aquatic and mostly marine, free-swimming and sedentary (tending to coral 



formation). 



Class I. — Hydrozoa. 



Types— Hydra; Obelia; 

 (TMbiilaria). 



I. Simple gastric cavity. 



2. No ectodermal gullet. 



3. Two phases, a free- 



swimming medusoid 

 and^ sedentary hy- 

 droid. 



MEDUSOID. 



A velum. 



Gonads ectodermal. 



Four radial canals. 



Simple sense-organs. 



Nerve-rings. 



HYDROID. 



1. When colonial, 



usually has horny 

 perisarc. 



2. Skeleton has no 



septa. 



Class II. — SCYPHOZOA. 



Types— Actinia; A ley on- 

 ium; Aurelia; 

 (Madrepora). 



1. Gastric cavity divided 



by mesenteries into 

 central and peripher- 

 al cavities, 



2. An ectodermal gullet. 



3. Two phases, a free- 



swimming medusoid 

 and sedentary hy- 

 droid. 



MEDUSOID. 



No velum. 



Gonads endodermal. 



Many radial canals. 



Tentacles modified in- 

 to complex sense- 

 organs. 



Diffuse nerve fibres. 



HYDEOID. 



1. When colonial, has 



calcareous skele- 

 ton. 



2. Skeleton has septa. . 



Class III.— Ctenophora. 

 TyPes — Cydippe; (Beroe). 



1. Gastric cavity consist- 



ing of stomach and 

 gastro - vascular 

 canals. 



2. An ectodermal gullet. 



3. One phase only, a free- 



swimming, modified 

 from medusoid type. 



4. Eight longitudinal 



rows of cilia. 



5. Nematocysts rare. 



6. Single aboral sense- 



organ. 



* Usually termed the "mouth.' 



