THE COELENTERATES AND CTENOPHORES 101 



Two tentacles arising in lateral ectodermal depressions or 

 pockets are characteristic of most representatives of this phylum. 

 At the aboral pole occurs a slight depression within which is 

 located a single otocyst. In the transverse plane this depression 

 is continued as two narrow ciliated areas called the polar plates. 



Body form is not constant throughout the group. Many indi- 

 viduals are ovoid, others have a somewhat shortened dorsoventral 

 axis, and still others are drawn out into a narrow band. 



The nettling cells characteristic of coelenterates are wanting in 

 the ctenophores. In their place structures called adhesive cells 

 occur upon the tentacles. 



Digestive System. — The mouth leads into a tube called the 

 stomach, but since it is ectodermal in origin it is more properly 

 a stomodaeum. This stomach opens into an entodermal sac, 

 the funnel. The stomach and funnel are both flattened with the 

 broad axis of one at right angles with that of the other. From 

 the funnel are given off laterally two perradial vessels each of 

 which divides dichotomously to form the four interradial vessels, 

 (Fig. 58 B). By another dichotomous division these form 

 the eight adradial vessels each of which communicates with 

 one of the meridional vessels directly underlying the rows of 

 combs. Each perradial vessel gives off a branch called the 

 paragastric canal, which runs parallel to the stomach and ends 

 blindly. At its aboral extremity the funnel gives off a funnel 

 vessel which, after forming two or four branches, proceeds to 

 the aboral pole and there empties through two or more openings 

 called the excretory pores. 



Development. — Ctenophores undergo direct development from 

 the fertilized egg. There is no alternation of generations. One 

 of the most interesting facts regarding the development of the 

 egg is that the materials for the formation of the adult structures 

 seem to be definitely arranged within the egg even before the 

 first cleavage. Evidence of this has been gained from the fact 

 that injury to or removal of a bit of the cytoplasm leads to the 

 formation of a ctenophore lacking in some organ or structure 

 such as the reduction in number of combs or loss of a part of the 

 digestive system. 



Classes and Orders. — On the presence of or lack of tentacles, 

 two classes are recognizable: the Tentaculata and the Nuda. Of 

 these, the former includes three orders and the latter a single 

 order, the Beroida. Members of the order Cydippida (Pleuro- 



