40 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



takes the place of a radial symmetry ; e.g. Venus' girdle 

 among the Cteiiophora. (See Fig. 16, page 46.) 



The Coeleiiterata fall into two well-marked 

 divisions, Hydrozoa and Aiitliozoa ; in the 



former the mouth is placed on a projecting oral cone, 

 while in the latter it is sunk below the level of the oral 

 circlet of tentacles, and the cavity developed from the 

 enteron, and separating its wall from the body wall, 

 is traversed by partitions (mesenteric septa) 

 (Fig. 11), of which a certain number extend across 

 the whole of the cavity, while others only project for 

 a shorter or longer distance into it. 



CCELENTERATA. 



A. Hydrozoa. The hydrozoa fall into two well- 

 marked divisions, in the first of which the medusa 

 form, when developed, always has an infolded rim of 

 the body running round the inner edge of the mouth 

 of the bell (\UiUtm). In consequence of the presence 

 of this fringe it may be spoken of as fh r^pqplftt> 

 division ; in it the sense organs are never protected 

 by any lid or cover, and they are therefore known as 

 the yaked-eed Medussa 



and as the generative sacs never form projecting 

 pouches, they are by some spoken of as f!yypf.orarpa 



I. Crpsp^fioJfr T^ft Craspedota fall into three 

 groups ; in the first the organism is always hydri- 

 form; or the nutrient persons are hydriform, and 

 the generative medusiform, or the organism is always 

 medusiform. They may, therefore, be called Hydro- 

 medusse. Examples of these are : Hydra, Cordylo- 

 phora, Hydractinia, Sarsia, Oceania. 



In the second group we have those colonies of 

 hydriform persons in which the common stem becomes 

 richly impregnated with calcareous salts, and they 

 therefore may be known as Hydroid Corals or Hydro- 

 coral liiiae. Such are Millepora and Stylaster. 



