PHYLUM III, CCELENTERATA 



The Coelenterata are aquatic, usually marine animals, with a 

 radial (instead of a bilateral) symmetry and with the mouth 

 opening into the digestive cavity (coelenteron) ; this latter 

 usually forms the entire body cavity. The mouth is nearly 

 always surrounded by tentacles, — hollow or solid outgrowths 

 of the body wall. A cross section of the body wall or of a ten- 

 tacle shows it to be composed of three layers, an outer layer of 

 cells (ectoderm), an inner layer of cells (endoderm) and between 

 these two a layer of a rather stiff structureless substance (meso- 

 glcea) inclosing few or no cells. These animals are provided 

 with special offensive weapons, the nettle-cells (nematocysts). 



Derivation of name. — Coelenterata > Greek koilos, hollow, 

 H- enter on, intestine. The entire cavity within the body walls 

 acts as a digestive cavity (like the stomach and intestine of higher 

 animals). 



The Coelenterata are divided into the following classes : — 



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A. Hydrozoa . io8 



B. Scyphozoa 121 



C. Anthozoa 122 



D. Ctenophora 139 



CLASS A, HYDROZOA (HYDROIDS AND MEDUS.^) 



Type of class, Sertularia puniila (living) (Figs. 39, 40, C). 



This is a colonial form with the individuals, — the polyps, 

 in sessile cone-shaped cups (hydrothecae) arranged in two oppo- 

 site rows along a stem. The polyp consists essentially of a 

 hollow, sac-like digestive cavity, surrounded at the top by a 

 single circle of tentacles, and in the midst of the tentacles an 



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