ECHINODERMATA. 



257 



Class III. — Ctenophora. 



The Ctenophora (as the Pleuro- 

 brachia, Cestum, and JBeroe) secrete 

 no hard deposit. They are trans- 

 parent and gelatinous, swimming on 

 the ocean by means of eight comb- 

 like, ciliated bands, which work like 

 paddles. The body is not contrac- 

 tile, as in the Jelly-fishes. They are 

 considered the highest of Ccelente- 

 rates, having a complex nutritive ap- Fig. 209.-A ctenophora (puu- 



n , n . robrachia pileus) ; natural 



paratus and a dennite nervous sys- size, 

 tern. 



Subkingdom III. — Echinodermata. 



The Echinoderms, as Star-fishes and Sea-urchins, are 

 distinguished by the possession of a distinct nervous sys- 

 tem (a ring around the mouth) ; an alimentary canal, com- 



Fig. 210 Forms of Echinoderms, from radiate to annulose type: a, Crinoids; &, 



Ophiurans; c, Star-fish; d, Echini; c, Holothurians. 



pletely shut off from the body-cavity, and having both 

 oral and anal apertures; a water-vascular system of circu- 



17 



