72 



INVEBTEBRATA 



CHAP. 



by the appearance of a groove in the stalk ; it can take place, as 

 Friedemann shows, at different stages in the development, either 

 before or after the loss of the tentacles. 



If food is scarce the crown separates as a free-swimming 

 organism termed an Ephyra, and the stalk slowly regenerates a 

 new crown ; but if food is abundant the process of the formation of a 

 new crown begins before the old crown has separated, and before it 

 is well under way a second groove appears below it, and a third 

 crown starts to develop; and by a repetition of the process the 

 Scyphistoma comes to look like a pile of plates, and is called a 

 Strobila. This process is known as strobilization, and in this way 

 one hydra-tuba can give rise to multitudes of Ephyrae (Figs. 52, 53). 

 The just liberated Ephyra is about an inch across the disc. The 

 wart-like sense-tentacles develop otoliths in their distal endodermal 



cells, and this distal mass of 

 endoderm becomes separated 

 from the rest. The Ephyra 

 does not attain the characters 

 of the adult Aurelia until it 

 has grown to a size of at least 

 inch in diameter. The 

 change in its shape, which 

 brings in the adult features, 

 consists in the slow growth of 

 adradial cushions, which are 

 situated between the bases of 

 the eight lobes of the Ephyra. 

 These cushions, by their growth, 

 gradually fill up the deep re- 

 entrant notches in the disc of 

 the Ephyra, and change its star- 

 like outline into the rounded 

 outline of the adult Aurelia. 

 As each of these cushions grows, a new endodermal pouch grows 

 out from the ring-sinus and extends into it. At the same time 

 each endodermal pouch, which already occupies a lobe of the 

 Ephyra, becomes trilobed at its distal extremity. The median 

 branch of these three-pronged forks goes to the wart developed from 

 the base of the hydra-tuba tentacle once situated there. This wart 

 develops into the sense-tentacle of the Aurelia (Fig. 55). The two 

 lateral branches of the forks go into the two folds forming the forked 

 extremity of the Ephyra -lobe. These forks persist in the adult 

 as the curtains which eventually form a hood for the sense organ 

 of the adult. The oral cone becontes more and more prominent and 

 forms the manubriuna of the adult. 



The genus Pelagia goes no farther than this stage. Each 

 adradial cushion develops a single long tentacle, and one only. 

 r>,,4- : Aurelia, just as is the case with the Hydromedusan 



FIG. 51. Two horizontal sections through the 

 upper part of a Hydra-tuba, about as old as 

 that represented in Fig. 50, to show the 

 formation of the ostium connecting the 

 stomach pockets. (After Friedemaun.) 



A, above the level of the ostium. B, at the level of 

 the ostium ; ost, ostium. 



But in 



