186 



ALTERNATIONS OF GENERATIONS. 



B 



rings, each of which eventually gives rise to a Medusa known 

 as an Ephyra (fig. 85). In each 

 of these rings is a dilation of the 

 stomach, and a section of each of 

 the four rudimentary mesenteries 

 described in connection with the de- 

 velopment of the Scyphistoma. As 

 the constrictions become deeper the 

 segments of the body between them 

 become disc-like, and their edges 

 are produced into eight lobes con- 

 taining prolongations of the gastric 

 cavity (fig. 85 C). The lower sur- 

 face of each disc, which forms the 

 future aboral surface of the Medusa, 

 becomes convex, in part owing to 

 the development of gelatinous tis- 

 sue. On the opposite surface a 

 muscular layer becomes developed. During the above process 

 the body of the Scyphistoma gradually grows in length and 

 continues to be segmented, so that a series of Ephyrae are 

 uninterruptedly formed, of which those near the base are the 



FIG. 85. THREE STAGES IN 

 THE ALTERNATIONS OF GENERA- 

 TIONS OF AURELIA AURITA. (From 

 Gegenbaur.) 



A. Polype stage. 



B. Commencing strobilization. 



C. Completed strobilization. 



youngest. The original terminal 



ring 



of tentacles of the 



Scyphistoma gradually atrophies. 



In the further development of the Ephyrae each of their eight 

 lobes becomes bifid at its extremity. 



As the Ephyrae successively reach this condition they be- 

 come detached, and by a series of remarkable changes, amount- 

 ing almost to a metamorphosis, and accompanied by an enor- 

 mous growth in size, reach the adult condition. 



The alternation of generations in the Acraspeda cannot be 

 quite so simply explained as in the Hydromedusae, though the 

 principle is probably the same in the two cases. 



Actinozoa. Amongst the Actinozoa there occurs in Fungia a 

 peculiar process which is, as shewn by Semper (171), in many 

 ways analogous to alternations of generations 1 . From the larva 

 a nurse-stock is developed, at the end of which a cup-like coral 



1 Vide also Moseley. Notes by a Naturalist of the Challenger, pp. 524 and 525. 



