96 



INVERTEBRATE MORPHOLOGY: 



medusan, then, it is evident that the medusa form is a secondary modifica- 

 tion a specialized reproductive organ, which in the Narcomedusas and 

 Trachymedusas has become so important that the ancestral polyp form is 

 practically suppressed in the life-history. On this view it must be sup- 

 posed that organisms so similar as the rueduscb of the Tubularian and 

 Campanulariau polyps have been developed entirely independently of one 

 another, a view which carries with it many difficulties, and that the medu- 

 soid buds represent stages in their evolution. 



It seems more probable, however, that, leaving the Hydrariie out of the 

 question, in all the other groups the medusa was the parent form. This 

 is borne out by the fact that in the Narco medusae, which, with their broad 

 pouch-like extensions of the gastric cavity, are the most primitive of all 



the craspedote medusae, there is no fixed 

 polyp form. It has been shown, how- 

 ever, that the Narcomedusae and Trachy- 

 medusae in their development pass 

 through a stage which may be considered 

 to represent the polyp form, and if, while 

 in this form, non-sexual reproduction 

 should have taken place, the buds re- 

 sembling the immature form which gave 

 rise to them, a polyp colony would result, 

 some of the buds of which might con- 

 tinue their development and become 

 medusae. By this view the difficulties 

 presented by the similarity of the medusae, 

 throughout all the groups where they 

 occur are overcome and the medusoid 

 buds are regarded as imperfectly devel- 

 oped or degenerate medusae. Further- 

 more this view is rendered more than 

 probable by the development of Ctinoc- 

 tantha aud the allied Cunina. The 

 former while in the embryonic polyp 

 form actually does bud (Fig. 49), the 

 buds resembling the original embryo which gave rise to them, and all the 

 buds, the parent embryo included, later develop into medusa?. In Cunina, 

 however, the parent embryo which gives rise to buds undergoes no further 

 development, only the buds continuing on their course of growth to medu- 

 sae. In this case a true aud typical alternation of generations occurs and 

 points out a simple explanation of the alternation which is found in the 

 Anthomedusae and Leptomedusae. In these the polyp colonies are the re- 

 sults of non-sexual reproduction of a larval medusa, and some only of the 

 individuals so formed continue their development to medusae. 



The relationships of the Hydrocorallinae to the other groups a:-e not yet 

 quite demonstrated. It would seem, however, from the medusa-bud which 



FIG. 49. BUDDING LARVA OF 



CunoctanUia octonaria (after 

 McCEKADY, from BROOKS). 

 a = egg larva. 

 bb = budded larvae. 



