vol.i.] Torrey. — Eydroida of the r<i<-ifi<- Const. 63 



( tbelia colony is to become a hydranth or a blastostyle. Hickson 

 ('91) has described in Millepora murrayi the transformation of 

 zooids into male medusae, brought about by the immigration of 

 sex cells from the coenosarcal canals. This case differs from 

 that of 8. Jialecina in that in the latter no direct transformation 

 of well formed hydranths into gonophores can be said to take 

 place, much less through the agency of sex cells. Yet the two 

 cases resemble each other in the substitution of one sort of indi- 

 vidual for another. In S. Jialecina the degeneration of the 

 hydranth does not appear to be due to the same cause which 

 initiates a gonophore. It is possible, however, that the conditions 

 which favor the degeneration of the one may determine the 

 growth of the other. But the solution rests with the experi- 

 menter. 



The following hydroids form a suggestive series with reference 

 to the origin of the gonosome: 



1. Obelia — in which the blastostyle is not preceded by any 

 degeneration, does not take the place of another sort of individual, 

 and is not determined by the presence of sex cells. 



2. Gampanularia — in which the blastostyle originates as in 

 Obelia, accompanied, however, by sex cells which appear in the 

 endoderm of the bud. 



3. Sertularella Jialecina — in which the formation of the 

 gonophore is preceded by the degeneration of a hydranth, though 

 no causal nexus is apparent, and sex cells seem to be absent. 



4. Millepora murrayi — in which the formation of the gono- 

 phore is preceded by the degeneration of a zooid, both being due 

 to the same cause — the presence of sex cells in the wall of the 

 zooid. 



In each case the function of the buds which may form blasto- 

 styles or gonophores is determined by internal or external con- 

 ditions. The problem is essentially physiological, and needs 

 experimental analysis. 



Sertularella hesperia, sp. nov. 



PI. VII. Figs. 57, 58. 



Trophosome. Stems with a few branches and a rambling habit, rising 

 from a creeping stolon to the height of 30 mm.; divided obliquely into 



