OF THE HYDROMEDUSJE. 407 



hydra like the parent and, like it, becomes directly converted into a medusa. As these 

 secondary hydras originate as buds, they are at first sessile, but they become detached 

 while in the hydra stage, or at least before they are completely converted into true me- 

 dusae: the time of detachment is not constant and although the larvae are at first sessile, 

 and, therefore, not actinuhe, they serve to show that the boundary line between a float- 

 ing actinula and a sessile hydra is an extremely faint one. 



Owing to the occurrence of asexual multiplication, each Cunina egg may give rise to 

 an indefinite number of adult medusa?, but as each larva becomes directly converted into 

 a medusa by a process of growth, there is no alternation and the life-history may be rep- 

 resented by the following diagram: 



Hydra = Medusa X <[ Egqs. 

 X 

 in. Cunina octonama Egg = Planula = Actinula — Medusa X <^Egqs. 



X 

 * Hydra = Medusa X <C Eggs. 



Here we have asexual multiplication without alternation, but in the Cuninae which 

 Uljanin and Metschnikoff studied there is a true alternation which is obviously of second- 

 ary origin and undoubtedly due to a very slight modification of such a life-history as the 

 one shown in diagram III. The planula itself is very peculiar and is furnished with an 

 anomalous pseudopodial apparatus for clinging to and fastening upon the gastric proc- 

 ess of the Geryonid within which it becomes a parasite; and the actinula, or primary hy- 

 dra, into which it becomes converted, never completes its development into a perfect, free 

 medusa. It remains as a brood-stock, from which other larvae are budded, and these are 

 set free and become converted into medusa? so that the life-history is represented by the 

 following diagram, in which for the first time, we find a true alternation: 



( Hydra = Medusa X <C Eggs. 

 IV. Cunina (Cunocantha) parasitica Egg— Planula = Actinula X j Hydra = Medusa x <C Eggs. 



^ Hydra = Medusa X <C Eggs. 



A comparison of Metschnikoff's account of the development of Cunina (Cunocantha) 

 parasitica, and that which I have given of Cunina octonaria, will bring out an interest- 

 ing and significant difference between them which I have not yet pointed out. In the 

 American Cunina, the hydra-stage is well marked in the larva 1 which are produced by 

 budding as well as in the one which hatches from the egg. In Metschnikoff's species, 

 however, the characteristics of the adult medusa begin to make their appearance in the 

 secondary buds, almost as soon as the buds themselves appear, and it would be difficult 

 to recognize a hydra-stage in the life of this species if we were not acquainted with the 

 simpler life-history of the American Cunina. In Metschnikoff's species, the primary hy- 

 dra is also greatly modified to fit it for its parasitic life, but in other respects its life- 

 history is very similar to that of the ordinary hydroids ; and if the acquisition of the 

 medusa characteristics by the secondary buds were a little more accelerated so that 

 their hydra characteristics were entirely, instead of almost, crowded out, we should have 

 a life-history like this: 



