CCELENTHKATA 



63 



nies generally have a skeleton, which is usually chitinous, 

 more rarely of calcium carbonate ; in either case it is secreted 

 by the ectodermal cells. 



The medusae are generally free-swimming animals, for the 



■ 



FlG. 46. Obelia sp. A, portion of a colony, certain parts in longitudinal section ; B, medusa ; 

 C, medusa with reversed umbrella; D, oral aspect of medusa. Bd. 1, 2, polyp-buds ; d/s, 

 blastostyle; coe, coenosarc ; ect, ectoderm; end, entoderm; cnt, coelenteron ; g.tk, gono- 

 theca; h.th, hydrotheca ; /, lithocyst or octocyst ; m.bd, medusa-bud; mnb, manubrium: 

 msgl, mesoglcea : mt/i, mouth; p, perisarc ; P. 1, 2, 3, 4, polyps; > dial canal; /. 



tentacle; vl, velum. (After Parker and Haswell.) 



