PHYLUM CCELENTERATA 163 



ridge appears a single gastric filament, soon to be followed by 

 others, and in the notches at the extremities of the eight arms 

 tentaculocysts make their appearance. In the meantime the 

 spacious enteric cavity is continued into the eight arms in the 

 form of wide radiating canals. 



As the ephyrula grows the adradial regions at first deeply 

 notched grow more rapidly than the rest, the result being that 

 the notches become gradually filled up, and the umbrella, from an 

 eight-rayed star, becomes a nearly circular disc. Four oral arms are 

 developed, and numerous marginal tentacles, and the ephyrula 

 gradually assumes the form of the adult Aurelia. It seems 

 probable that the sub-genital pits of the medusa are formed 

 from sections of the septal funnels of the Scyphula. 



Thus the life-history of Aurelia differs in several marked 

 respects from that of any of the Hydrozoa. There is an alternation 

 of generations, as in Obelia, the gamobium being represented by 

 the adult Aurelia, the agamobium by the Scyphula. But instead 

 of the medusa being developed either as a bud on a branched 

 colony, as in Leptolinse, or by direct metamorphosis of a polype, 

 as in Trachylina?, it is formed by the metamorphosis of an ephyrula 

 developed as one of several transverse segments of a polype. 



It Las been shown that, under exceptional circumstances, the 

 egg of Aurelia develops directly i.e. without the interposition of 

 a Scyphula-stage into the adult medusa. As we shall see, this is 

 the normal mode of development of many allied forms. 



2. GENERAL STRUCTURE AND CLASSIFICATION. 



The Scyphozoa may be defined as medusoid Coelenterata, having 

 the same general structure and arrangement of the layers as the 

 medusoid Hydrozoa, but differing from them in the possession of 

 endodermal gastric tentacles; in having endodermal gonads dis- 

 charging their products into the digestive cavity ; and, in nearly 

 all cases, by the absence of a velum, and in the presence of sense- 

 organs in the form of hollow sense-clubs or tentaculocysts. How 

 far a stomodseum or ectodermal gullet is characteristic of the 

 group is uncertain. As in the Hydrozoa, the medusa develops 

 directly from the egg in some species, while in others there is an 

 alternation of generations, a polype -form (agamobium) giving rise 

 to the medusa-forij^(gamobium) by a process of transverse fission. 

 In the majority of cases, however, nothing is known of the life- 

 history, the process of development having been worked out only 

 in a few cases. 



As far as is known the segmenting embryo gives rise to a gastrula 

 by invagination : by the closure of the blastopore a planula is 

 produced, at one end of which a second invagination takes place, 

 forming the stomodseum. 



M 2 



