TTPB C(ELENTERA. 103 



the substance of the lobes, and uniting finally in a larger 

 central canal at the upper extremity of which lies the original 

 mouth. 



Development of tJw Scyphomedusce. — The segmentation of 

 the ovum leads as usual to a blastula form which may be- 

 come solid by immigration and subsequently hollow out, or 

 may abbreviate these processes by a typical invagination. 

 The gastrula resulting passes in some forms gradually into the 

 adult condition (Pelagia), without ever relinquishing its free- 

 swimming habits, though in the majority of cases in which the 

 life-history is known, cases confined entirely to the Discome- 

 dusse, the embryo settles down and leads for a time a fixed 

 sessile existence, resembling very much a polyp. This polyp 

 form (Fig. 55, A), known as the ScypJiostoma, differs mate- 

 rially from the hydroid polyp ; in the first place, from the 

 body-wall there project into the coelenteron four linear 

 ridges (mesenteries) which extend from the neighborhood of 

 the mouth to the posterior end of the coelenteron, and later 

 give rise to the mesenterial filaments of the medusae ; and in 

 the second place, a deep funnel-like depression extends into 

 each one of these mesenteries from the oral surface of the 

 Scyphostoma, and produce later the subgenital chambers of 

 the medusae. 



The Scyphostoma may develop directly into the free- 

 swimming medusae, but in a number of forms it undergoes a 

 series of transverse divisions (Fig. 55, B) into a number of 

 saucer-like structures produced at the edges into eight blunt 

 notched prolongations. These separate from the parent Scy- 

 phostoma and are known as Ephyrce (Fig. 55, G), developing 

 into the adult Discomedusan by the intervals between the 

 eight lobes, which carry the sense-organs, filling out, by the 

 development of tentacles, and by the growth of the coelenteron. 

 A typical alternation of generations is thus brought about. 



There can be little doubt but that the ancestral Scyphomedusan was a 

 sessile organism with much resemblance to Lucernarla. From this two 

 lines of descent arose, both marked by a development of sense-organs from 

 tentacles and terminating in the Peromedusae and Oubomedusae respec- 

 tively, the Discomedusae resulting in the culmination of this sense-organ 

 development. So far as the DiscomedusEe themselves are concerned the 



