Some of the Lozver Metasoans 399 



gonads are located in the walls of the canals of the gastrovascular system, 

 and the mature cells are discharged into the water through the mouth. 

 Development is direct and there is no alternation of generations. In 

 many, a larval form resembling adults of some of the more primitive 

 ctenophores develops. 



The Classes of Ctenophores. — There are only two classes of 

 ctenophores which are as follows : 



Class I. Tentaculata. These forms have tentacles. Most of the cteno- 

 phores are in this class. Example : Pleurobrachia. 



Class II. Nuda. These fornis are without tentacles. Example: Beroe. 



_SENSE ORGAN 



Fig. 132. — Examples of ctenophores. A, Pleurobrachia; B, Cestum, the 



Venus's girdle; C, Beroe. 



THE CLASS TENTACULATA 



Pleurobrachia is a common example of this group. It is an ovoid, 

 small form, white or rose-colored, with long tentacles. Its length is 

 about 2 cm. with the tentacles being about 15 cm. long. It is distributed 

 from Long Island to Greenland, in Europe, and on the Pacific Coast. 



The mouth at the oral end of the animal opens into the pharynx 

 which then opens into the stomach. From the stomach digestive canals 

 branch beneath the comb plates. The sense organ is located at the aboral 

 end, and the eight comb plates extend from the region of the sense 

 organ toward the mouth. The tentacles extend from two blind sacs 

 located near the aboral end. 



While swimming, the tentacles are extended outward in a looping 

 manner, better to encounter any possible microscopic Crustacea, mollusc 

 larvae, or even protozoans. These are captured by means of the sticky 

 cells covering the tentacles and are then conveyed to the mouth region 



