PHYLUM CTENOPHORA 173 



an asexual generation, is here developed as it is nowhere else 

 in the animal kingdom. 



PHYLUM CTENOPHORA 



The ctenophores, comb-jellies, or sea- walnuts (Fig. 72), 

 are extremely transparent, jelly-like animals which swim 

 at the surface of the pcean. They are often shaped like a 

 walnut, as one of their names indicates, and have two re- 

 tractile tentacles which trail behind as they swim. There 



FIG. 72. A ctenophore. 



are eight rows of comb-like paddle plates which propel the 

 body slowly through the water. Ctenophores show a 

 slight superiority over coelenterates in the structure of the 

 digestive system by having an opening, the anus, at the 

 opposite end from the mouth. They are usually protected 

 from enemies by their extreme transparency, which renders 

 them practically invisible during the day. Many species 

 are phosphorescent, and give off light when disturbed. 



