DEVELOPMENT OF AURELIA 



189 



The gradual widening of the central cavity renders the gullet tube 

 less obvious, and results in an increasing resemblance to the medusa 

 type. 



In late autumn, however, a more fundamental change occurs in the 

 history of the Hydra-tuba, (a) Occasionally, by a " telescoping," 

 the Scyphistoma becomes detached 



^M° 



Fig. 96. 



—Lucernaria. — After 

 Korotneff. 



and converted into a free-swim- 

 ming Eph>T:a, which in turn be- 

 comes a jelly-fish, {b) Sometimes, 

 in unfavourable conditions, a 

 furrow appears roimd the upper 

 region of the Scyphistoma, the 

 upper portion is converted into 

 an Ephyra, and floats away, while 

 the lower portion re-forms its oral 

 region by regeneration, and pro- 

 duces another Ephyra. (c) In 

 ordinary conditions the Scyphi- 

 stoma elongates, and displays a 

 succession of annular constric- 

 tions. This stage, often compared 



to a pUe of discs or saucers, is called a Strobila. Each disc is separated 

 off in its turn as a free-swimming Ephyra, which becomes a jelly-fish. 

 The still undivided basal portion may rest for a time, and then 



undergo further constric- 

 tion. This is probably 

 an abbreviation of the 

 primitive mode of de- 

 velopment. 



In the conversion of the 

 Scyphistoma into the 

 Eph\T:se, the diverticula 

 coalesce into a general 

 cavity, the entrances to 

 the septal invaginations 

 probably persist as the 

 sub -genital pits, the 

 gastric filaments sprout 

 out from the remains of 

 the septa, and so mark 

 the place where the ecto- 

 dermal gullet passed into 

 the endodermal cavity. 



The first Ephyra differs 

 from those which come 

 after it in bearing the original tentacles of the Hydra-tuba. From its 

 margin eight bifid lobes grow out, each embracing the base of a perradial 

 or interradial tentacle. The bases of these eight tentacles become the 

 sense organs or rhopalia. The other eight adradial tentacles atrophy. 

 On the Ephyrae which follow there are at first no tentacles, only the 

 eight bifid marginal lobes which bear the sense organs in their niches. 



Fig. 97. 



-Diagram of Lucernaria. — 

 After AUman. 



C, Cavity of gut (ccelenteron) ; F., gastric fila- 

 ments ; H., hypostome ; G., gonad ; T., tentacle ; 

 C.C., circumference canal. 



