HYDROMEDUSAE 



99 



the coelenteric cavity is again, patent forming the ring canal into which 

 the radial canals open at their outer ends (Fig. 37, r). 



The medusa is the sexual phase in the life-history of Obelia. It 

 possesses gonad (Figs. 37 and 41,^) in the form of testes or ovaries which 

 form four conspicuous pear-shaped or rounded masses hanging down 

 from the lower (concave) surface of the umbrelkj immediately beneath 

 the radial canals. Each is situated, as in the case of Hydra, in the 

 thickness of the ectoderm. 



Obelia affords an excellent example of alternation of generations — 

 generations of sexual individuals (in this case Medusae) being intercalated 

 amongst others which are not sexual (in this case the individuals of the 

 colony). 



TUBULARIA 



It is interesting to compare with Obelia another common marine 

 Hydrozoon — Tubularia. 

 Here again colonies are 

 formed by a process 

 of budding, consisting of 

 hydroid individuals which 

 however differ from those 

 of Obelia in various details, 

 some unimportant, some 

 important. Amongst the 

 former are the much 

 greater size of the polyps, 

 and the fact that a second 

 set of smaller tentacles 

 are present situated close 

 to the tip of the oral 

 cone and immediately 

 surrounding the mouth 

 (Fig. 40). The most 

 important fact is that 

 in Tubularia there is no 

 hydrotheca surrounding 

 and protecting the body 

 of the polyp, the horny 

 perisarc (j)s) being re- 

 stricted to its cylindrical 

 stalk and stopping short of the swollen body of the polyp. Such a 



H I 



ps. 



Fig. 40. 

 Tubularia, upper portion of a single polyp, x 17. 

 M, young medusa bud ; o.c, oral cone ; ps, perisarc. 



