OF THE HYDROMEDUSiE. 409 



As I have shown, this secondary alternation occurs in many hydroids, such as Hy- 

 dractiuia, Eutima, Turritopsis, Obelia (Merejkowsky) and others, and it was correctly 

 described by Wright in Hydractinia in 1856; hut, 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. It is certainly a secondary acquisition, as we may see from the fact 

 that in Tubularia, Eudendrium and other hydroids, the planula becomes directly con- 

 verted into a hydra. So Car as this point is concerned, the life-history of Eutima or Hy- 

 dractinia, and that of Tubularia or Eudendrium present the following contrast. 



Hydra 

 X 

 Tubularia Egg = Planula = Actinula = Hydra 



X 



Hydra 



with no alternation, while in the other forms we have 



( Hydra 

 Eutima Egg = Planula = Root X \ Hydra 



£ Hydra 



with an alternation. 



The complete life-history of Eutima with its double alternation between the root and 

 the hydranths, between the hydranths and the medusa?, and its polymorphism, and divi- 

 sion of the hydranths into nutritive persons and blastostyles, may be represented as fol- 

 lows: 



f Medusa <^ Eggs. 



' Nutritive Hydra X 



VIII. Eutima Egg = Planula = Root X -\ 



Blastostyle X { Medusa <^ Eggs. 



I 



(^ Medusa <^ Eggs. 



^ Nutritive Hydra 



{ Nutritive Hydra 



I I 



(^ Nutritive Hydra X -J { Medusa <^Eggs. 



[Blastostyle X ■{ Medusa <^Eggs. 



Medusa <^Eggs. 



I 



In Podocoryne (Dysmorphosa) we have an extremely complex life-history which, 

 however, is readily derivable from one like that of Eutima as just given. There is a sec- 

 ondary alternation between the root and the hydranths as in Eutima, and the polymorph- 

 ism between the hydranths is more specialized, as we find not only nutritive polyps and 

 blastostyles but defensive polyps as well; and as each medusa, in addition to its sexual 

 function, also possesses the power to produce other medusae by budding, the number of 

 sexual animals which may be derived from a single egg is unlimited. 



The following diagram represents the life-history of this species, except that the first 

 generation of medusa', like the second, gives rise to reproductive elements. 



