OELENTERATA. 



159 



developed, and the whole embryo assumes a medusiform character. 

 Fresh tentacles arise, the velum and cavity of the umbrella become 

 established, but these changes do not involve any points of very 

 special interest. 



Siphonophora. The development of the Siphonophora has 

 been the subject of careful investigation by Haeckel (158) and 

 Metschnikoff (163). The ova are large and usually (except 

 Hippopodius) without a membrane. 



They are formed of a peripheral denser layer of protoplasm 

 and a central spongy mass. They usually undergo their entire 

 development in the water. In some instances they have been 

 successfully reared by artificial impregnation. 



As an example of the Calycophoridae I shall take Epibulia 

 aurantiaca, a form allied to Diphyes, the development of which 

 has been studied by Metschnikoff 1 . 



FIG. 74. THREE LARVAL STAGES OF EPIBULIA AURANTIACA. (After Metschnikoff.) 



A. Planula stage. 



B. Six-days' larva with nectocalyx (nc) and tentacle (/"). 



C. Somewhat older larva with gastric cavity. 



ep. epiblast; hy. hypoblast; so. somatocyst; nc. nectocalyx; /. tentacle; c. large 

 yolk cells ; po. polypite. 



1 In my description of the development of the Siphonophora I employ Huxley's 

 terminology. 



