CGELENTER ATA HYDROZOA. 133 



section be made of an actinozoon, a double tube will have 

 been dissected. " If you could suppose the stomach of a 

 hydrozoon thrust into that part of the body with which 

 it is continuous, so that the walls of the body should rise 

 round it and form a sort of outside case, containing a 

 prolongation of the general cavity, the hydrozoon would 

 be converted into an actinozoon.." (Huxley). 



190. In the compound Hydrozoa, the individual zooids 

 which form a colony are denominated polypites, to dis- 

 tinguish them from the polypes in the Actinozoa, and 

 the polypides in the Polyzoa. The entire body of a 

 hydrozoon, whether simple or compound, is called the 

 hydrosoma (Gr. hudra-, and soma, body) ; the substance 

 which is common to the various polypites in a colony, the 

 coenosarc. 



The compound Hydrozoa (sertularia, &c.,) were formerly 

 placed along with the Polyzoa, both of which were 

 popularly called Corallines. These two groups are, how- 

 ever, widely different in internal structure. 



191. (1.) The Polyzoa possess an alimentary canal, 

 which is completely shut off from the general cavity of 

 the body, and communicates with the exterior by an anal 

 opening. The alimentary tube, in the Hydrozoa, opens 

 directly into the body cavity, and is not separated from 

 the body walls. 



(2.) The Polyzoa have a distinct nervous system. To 

 nerves have been found in any hydrozoon. , 



(3.) In the Polyzoa the reproductive organs are internal. 

 In the Hydrozoa they form " outward processes of the 

 body wall, and are directly in contact with the surround- 

 ing medium." 



192. The fresh-water hydra may be taken as an example 

 of this class. This animal is common in ponds, and may 

 be found attached to the stems of aquatic plants. In its 

 contracted state it is a little mass of green jelly, about 

 half the size of a pea. If this little gelatinous ball is 

 placed in a glass of water it will manifest its true form. 

 The body is a tube, about half an inch long, and not much 



