C(ELENTERATA. 



131 



solid hypoblastic axis (fig. 73). At this stage the larva closely re- 

 sembles the larva of Tubularia. An alimentary cavity is hollowed out in 

 the centre of the hypoblast which soon opens by a wide oral aperture (m). 

 A second pair of arms becomes formed, which are at first much shorter 

 than the original pair ; with their formation a radial symmetry is 

 acquired. Sense-organs become at the same time 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 CalycophoridaR I shall take Epibulia 

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

 been studied by Metschnikoff 1 . 



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



A. Planula stage. 



B. Six-days larva with nectocalyx (nc) and tentacle (t). 



C. Somewhat older larva with gastric cavity. 



ep. epiblast; liy. hypoblast; so. somatocyst; nc. nectocalyx; t. tentacle; c. large 

 yolk cells ; po. polypite. 



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

 terminology. 



92 



