136 SYDNEY J. HICKSON. 



Examples : Clytia, Tiara, Rathkea, Obelia, Tima, 

 ^quorea (Metschnikoff, 42), and Cyansea arctica 

 (McMurrich, 41). 



b. Intermediate forms, in which the migration takes 



place mainly at the hind end, occur in Mitrocoma 

 (MetschnikoflF, 42). 



C. A sterrula is formed by polypolar immigration of cells 



into a large segmentation cavity, these cells being 

 formed by the radial fission of the cells of the cceIo- 

 blastula. 



Example : ^Eginopsis (Metschnikoff, 42). 



c. Intermediate form, in which the cells that immigrate 



are formed partly by radial and partly by tangential 



division. 



Example: Hydra (Brauer, 5). 



D. A planula is formed by primary delamination, the 



endoderm-cells formed by tangential division only. 

 The segmentation cavity is large. 



Example : Geryonia (Metschnikoff, 42). 



d. Numerous intermediate forms in which the segmen- 



tation cavity is small. 



Examples: Tubularia (Brauer, 5a), Bougainvillea 

 (Gerd, 14). 



E. A sterrula is formed by precocious delamination 



(secondary delamination of Metschnikoff). No seg- 

 mentation cavity formed. 



Examples : Aglaura (Metschnikoff, 42), Rhopalo- 

 nema (Metschnikoff), Eudendrium and Sertularella 

 (Tichomiroff, 55). 



e. Intermediate forms in which the segmentation is at 



first incomplete. 



Examples: Renilla (Wilson, 63), Gorgonia (von 

 Koch, 36), and probably other Alcyonarians. 



F. A multinucleated plasmodium is formed. There is no 



segmentation and no segmentation cavity. 



Examples : Algaphenia (Tichomiroff, 55), Mille- 



