Metagenesis among the Hydromedusm. 27 



{Hydra X \^'f''''^^J<^'- 

 IvRRiTOFBis: rffff = rianu'a= Root X < Hydra X \ Medtisa^Eggs. 



Hydra X 3^^f««<|W*-- 



In the orcliiiaiy Campanularians, with free medusa?, we 

 have a new element of complexity, owing to the appearance 

 of polymorphism. The ordinary hydras no longer give rise 

 to medusa-buds, and these are produced only on the repro- 

 ductive hydras or blastostyles. In Eatima^ which I shall 

 take as an example of this group, we have another complica- 

 tion which is very significant. 



As in T'lirritopsis^ there is a secondary alternation of gene- 

 rations, for, as I have shown above, the planula no longer 

 becomes converted into a hydra, but forms a root from which 

 the primary hydra is budded like those which appear later. 



As I have shown, this secondary alternation occurs in many 

 hydroids, such as Hgdractinia^ Eutimaj Tiirritopsis^ Ohelia 

 (Merejkowsky) , and others, and it was correctly described by 

 Wright in Hydractinia in 1856 ; but, so far as I am aware, 

 no one has pointed out that it is a true alternation, exactly 

 like the alternation between the hydra and the medusa, and 

 that it is certainly a secondary acquisition, as we may see 

 from the fact that in Tabularia^ Eudendriuni^ and other 

 hydroids the planula becomes directly converted into a hydra. 

 So far as this point is concerned, the life-history of Eutlma or 

 Hydractinia and that of Tabularia or Eudendriam present 

 the following contrast : — 



Hydra 

 X 

 TuBULARiA : Egy = Planula = Adinula = Hydra 



X 

 Hydra 



with no alternation, while in the other forms we have 



C Hydra 

 Eutima: Egg=Plumda=:Rootx < Hydra 



(^ Hydra 



with an alternation. 



The complete life-history of Eutima, with its double alter- 

 nation between the root and the hydranths, between the 

 hydranths and the raedusai and its polymorphism, and division 

 of the hydranths into nutritive persons and blastostyles, may 

 be represented as follows : — ■ 



