ECHINODERMATA. 



257 



CLASS III. Ctenophora. 

 The Ctenophora (as the Pleuro- 

 Irachia, Cesium, and Beroe) 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 Coelente- 

 rates, having a complex nutritive ap-FiG.209._Acteno P hore<p?e- 



, ,, . robrachia pileus) ; natural 



paratus and a dennite nervous sys- size. 

 tern. 



Subkingdom III. ECHINODEKMATA. 



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- 



Jl 



PIG. 210. Forms of Echinoderms, from radiate to annnlose type: a, Crinoids; ft, 

 Ophiurans; c, Star-fish; d, Echini; e, llolothurians. 



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

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



17 



