198 SYDNEY J. HICKSON. 



Comparing the medusa of Millepora with the adelocodonic 

 gonophore (fig. 4) of HydromedussB, the following points of 

 difference may be observed : 



There is a codonostome in the former, there is none in the 

 latter. 



In the former the endoderm extends almost to the margin 

 of the umbrella, in the latter the endoderm is reduced to a 

 shallow cup surrounding the base of the manubrium. 



In other respects the two gonophores are practically similar. 



Comparing the adelocodonic gonophore (fig. 4) with the male 

 gonophore of AUopora (fig. 5), two points of difference may be 

 observed. In the first place the endoderm completely surrounds 

 the gonad in the latter, excepting at a small aperture at the 

 distal pole, where it forms the inner wall of a narrow seminal 

 duct. Secondly, there is no layer of ectoderm between this 

 endoderm and the gonad in Distichopora. In the adelocodonic 

 gonophore there are two layers of ectoderm between the gonad 

 and wall of the gonangium. 



The male gonophore of Distichopora (fig. 6) resembles that 

 of AUopora (fig. 5) in all respects except one, namely, that in 

 the former there is no manubrium. 



The female gonophores of the two genera of Stylasteridse 

 resemble the male gonophores in most respects, but in the 

 former there is a more complicated plieting of the base to form 

 a nourishing disc (trophodisc), and no structure corresponding 

 to a manubrium can be observed. 



Do these gonophores of the HydrocorallinsB represent stages 

 in the degeneration, or do they represent stages in the evolu- 

 tion of the free medusiform gonophore ? 



It would be more satisfactory, perhaps, to leave these 

 questions to be answered at a time when we are better ac- 

 quainted with the minute anatomy of the gonophores of 

 other species of Millepora and the other genera of the Sty- 

 lasteridse. 



The very convincing proofs that have been brought forward 

 by Balfour, Weismann, and others, showing that the gonophores 

 of the Hydroidea, however simple in structure, represent stages 



