Metagenesis among the Hydromedusce, 25 



they become detached while in the hydra stage, or at least 

 before they are completely converted into true medusa3. The 

 time of detachment is not constant, and although the larvai 

 are at first sessile, and therefore not actinulas, they serve to 

 show that the boundary-line between a floating actinula and 

 a sessile hydra is an extremely faint one. 



Owing to the occurrence of asexual multiplication, each 

 Cunina &g^ may give rise to an indefinite number of adult 

 medusee ; 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 represented by the following dia- 

 gram : — 



Hydra := Medusa<zEggs. 



X 

 III. Cunina octonaria: Egg=Planula=Actimda=Medusa<^Eggs. 



X 

 Hydra= Medusa-<:.Eggs. 



Here we have asexual multiplication without alternation ; 

 but in the Cuninas which Uljanin and Metschnikoff studied 

 there is a true alternation which is obviously of secondary 

 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 p^eudopodial apparatus for clinging to and fasten- 

 ing upon the gastric process of the Geryonid within which it 

 becomes a parasite ; and the actinula or primary hydra into 

 which it becomes converted never completes its development 

 into a perfect free medusa. It remains as a brood-stock, from 

 which other larva3 are budded, and these are set free and 

 become converted into medusae, so that the life-history is 

 represented by the following diagram. In which for the first 

 time we find a true alternation : — 



IV. Cunina (Cunocantha) parasitica : j Hydra = Medusa<zEgys. 



Egg =Pla}iula = Actinula X \ Hydra = Medusa<iEgys 



\ Hydra= Medusa'^Eggs. 



A comparison of Metschnikoflf's account of the development 

 of Cunina [Cunocantha) ijarasitica and that which I have 

 given of Cunina octonaria will bring out an interesting and 

 significant difference between them which I have not yet 

 jwiuted out. In the American Cunina the hydra-stage is 

 well marked in the larvae which are produced by budding as 

 well as in the one which hatches from the e^^. In Metsch- 

 nikoflf's species, however, the characteristics of the adult 

 medusa begin to make their appearance in the secondary buds 



