Ill] CLASSIFICATION 83 



Class II. ACTINOZOA. 



Solitary or colonial Coelenterata with gullet lined by ectoderm : 

 coelenteron provided with radiating mesenteries: genital cells de- 

 rived from endoderm. 



Order 1. Alcyonaria. 



Eight mesenteries and eight fringed tentacles : spicules 

 in the jelly. Ex. Alcyonium, Tubipora, Corallium. 



Order 2. Zoantharia. 



Mesenteries usually in pairs, either six pairs or some 

 multiple of six: tentacles conical: no spicules but often an 

 external calcareous skeleton formed by the ectoderm. 



Class III. SCYPHOZOA. 



Coelenterata with alternation of generations : mesenteries present 

 in young, but later becoming absorbed : oral part breaks loose and 

 becomes developed into a free-swimming organism externally resem- 

 bling a medusoid : the stalk of the original polyp reproduces the 

 lost parts. Ex. Aurelia. 



Class IV. CTENOPHORA. 



Very widely different from the two preceding divisions : free- 

 swimming animals with a sense organ and nervous disc of skin at 

 the pole opposite the mouth : swim not by muscular contractions 

 but by vibrations of eight longitudinal bands of cilia radiating from 

 nervous disc, which bands consist of successive transverse rows of 

 cilia, the cilia of each row fused at base so as to form a comb-like 

 structure : only two tentacles, a gullet lined by ectoderm : stomach 

 represented by a system of branching tubes. Ex. Hwmipkora, 

 Cesium. 



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