106 SYNOPTIC COLLECTION. 



results in the formation of a blastula. Cells then migrate 

 into the interior and afterward arrange themselves as an 

 incomplete layer on the inner side. Here, then, in Cyanea 

 we have the stage following the blastula produced by the 

 immigration of cells and not by the invagination of a layer. 

 The parenchymella, after swimming about freely, settles 

 down and forms a cyst. Soon after leaving the cyst a 

 mouth opens and four tentacles are developed. 1 



The development of Aurelia illustrates that of most 

 Discophora. PL 134, fig. i, represents one of the early 

 stages of the egg, and fig. 2, the fully grown egg. Seg- 

 mentation takes place and when the embryo leaves the 

 parent it swims about by means of cilia (fig. 3). Grad- 

 ually the two layers are formed (fig. 4) , then the digestive 

 cavity (fig. 5). Afterward a depression in the outer sur- 

 face of the inner wall marks the position of the future 

 mouth (fig. 6) ; in time the outer wall is pierced and the 

 mouth and passage leading to the digestive cavity appear. 

 The embryo becomes elongated (fig. 7) . It now gives up 

 its free life. According to Prof. Louis Agassiz, " it settles 

 down upon its narrow end ; it wavers, and sways to and 

 fro as if it were trying to force its way downward into the 

 substance upon which it has fastened itself, and then, as 

 if dissatisfied with the promise of a good basis for its foun- 

 dation, it suddenly loosens its hold and swims away to 

 another locality, there to repeat the same kind of examina- 

 tion until finally, after perhaps half a dozen attempts .... it 

 finds a suitable place to rest upon permanently." The 

 changes which take place at this time are more clearly 

 seen in figs. 8-1 1 which are taken from the development 

 of Cyanea already briefly described. Fig. 8 represents 

 the embryo with a chitinous base which serves to 

 strengthen its attachment. In fig. 9 the two tentacles 

 are just beginning to grow. Fig. 10 has four tentacles 

 (well provided with thread cells) and a wide opened 



iSee Smith, Bull. Mus. Comp. Zool., XXII, 1891, p. 124. 



