COELENTERATA 



59 



Occasionally one finds a modified hydranth which is for the purpose 

 of reproduction, and is called the gonangium. 



The perisarc is the thick outside covering, which is hard and 

 chitinous. It is expanded to form the cup of the hydranth and is 

 then called the hydrotheca^ or in the case of the gonangium^ the 

 gonotheca. A shelf across the base of the hydrotheca is the support 



Fig. 23. Alternation of generations in the //>'^ro/iO/^f//«. (From Shumway, Gf wra/ 

 Biology. Courtesy of John Wiley & Sons.) 



for the hydranth. The soft parts of the stem are called the coenos- 

 arc, and the cavities of the coenosarc open into the hydranth forming 

 the typical gastrovascular cavity. 



The ectoderm contains nerve cells, epitheliomuscular cells and 

 interstitial cells with nematocysts of two or more types. The 

 mesoglea is an undifferentiated, jelly-like layer between the ectoderm 



