iv PHYLUM CCELENTERATA 175 



of transverse fission (G), becoming divided by a series of constric- 

 tions which deepen until the polype assumes the appearance of a 

 pile of saucers, each with its edge produced into eight bifid lobes, 

 four per- and four inter-radial. Soon the process of constriction 

 is completed, the saucer-like bodies separate from one another, 

 and each, turning upside down, begins to swim about as a small 

 jelly-fish called an ephyrula (H, I). The umbrella of the ephyrula 

 is divided into eight long bifid arms (a) with deep (per-radial or 

 inter-radial) notches: it has of course carried away with it a 

 segment of the stomach with the gastric ridges of the scyphula : 

 during the process of constriction this becomes closed in on the 

 proximal or ex-umbrellar side, while on the sub-umbrellar side it 

 remains open, and its edges grow out to form a manubrium. 

 Round the margin there are the bases of eight per-radial and 

 inter-radial tentacles, each in the notch of one of the arms, and 

 eight ad-radial tentacles in the intervals between the lobes : the 

 latter disappear completely; the former may persist as the 

 tentaculocysts. On each gastric ridge appears a single gastric 

 filament, soon to be followed by others, and in the notches at the 

 extremities of the eight arms tentaculocysts are now recognisable. 

 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-ra} r ed 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; in a sense, 

 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 LeptolinaB, or by direct metamorphosis of a 

 polype, as in Trachylinse, it is formed by the metamorphosis of an 

 ephyrula developed as one of several transverse segments of a 

 polype ; so that the life -history might be described as a metamor- 

 phosis complicated by multiplication in the larval (scyphula) 

 condition, rather than a true alternation of generations. 



It has been shown that, under exceptional circumstances, the 

 egg of Aurelia develops into scyphulse which do not undergo 

 transverse division, the entire scyphula becoming metamorphosed 

 into a single adult. 



