DEVELOPMENT OP DISTIOHOPOEA VIOLAOBA. 51 



development, and no one of them seems to be particularly 

 predominant. 



The development of a gastrula by invagination probably 

 occurs only in the group of Scyphomedusse. 



The formation of a planula by delamination (i. e. the primary 

 delaminatiou of Metschnikoff) occurs only in the group of the 

 Geryonidse. 



The formation of a sterrula by secondary delamination 

 occurs in most of the Anthozoa (McMurrich) and in many of 

 the Hydroids. 



The formation of a sterrula by hypotropic invagination 

 occurs in many Sertularidse and Campanularidae. 



The formation of a planula or sterrula by polypolar immi- 

 gration of cells into a hollow blastula occurs in a few forms. 



Lastly, the formation of a multinucleated plasmodium 

 without segmentation, which is followed by the differentiation 

 of epiblast-cells at the periphery of a solid plasmodium (the 

 endoderm), occurs in the Hydrocorallinse and in some Al- 

 cyonarians. 



Between these various types of development many inter- 

 mediate forms have been found, so that we have as it were a 

 complete series of developmental histories, with the typical in- 

 vaginate gastrula at one end and the multinucleated plasmo- 

 dium at the other. 



We may represent this series by the following plan : 



A. Gastrula formed by invagination. Large segmentation 



cavity. 



Examples : Cotylorhiza (Claus, 8), Pelagia nocti- 



luca, and Nausithoe (Metschnikoff, 42). 

 a. Intermediate forms between type A and B are found 

 in Aurelia flavidula (Smith, 52), in which the 

 clump of cells that are invaginated is at first solid, 

 and in Cyanaea capillata (McMurrich, 41), in 

 which this clump of cells remains solid longer 

 than in A. flavidula. 



B. A solid planula (sterrula) formed by hypotropous im- 



migration of cells into a large segmentation cavity. 



