408 w - K - BROOKS ON THE LIFE-HISTORY 



Medusa X <C Eggs. 

 V. Egg = Planula = Actinula X "j Medusa X <CEggs. 



X < £0gw. 



C Medusa 

 < Mediisa 

 C Medusa 



I know of no hydra which presents this life-history without modification, but there are 

 many Campanularians and Tubularians in which the only modification is the acquisition 

 by the actinula or primary hydra of the power to produce, in addition to the buds which 

 become medusa', other buds which remain in the hydra condition, and share with their 

 parent, the primary hydra, the power to produce both kinds of buds. Thus in Perigo- 

 nomus (Stomatoca), the egg gives rise to a planula which becomes the first hydra, and 

 this produces other hydras like itself and builds up a hydroid cormus; and ultimately 

 all these hydras give rise to buds which become directly converted into medusae, the hy- 

 dra-like stage being completely suppressed, and we have a life-history like this : 



£ Medusa X < Eggs. 

 Hydra X ^ Medusa x <E ggs. 



„ X . .. < Medusa X <Eggs. 



Hiidra X < ,, , JT„ 



x I Medusa X < Eggs. 



... „ „, . . ,. , n . rr , . . <, Medusa X <Eggs. 



VI. Erm = Plamua=Actimtta or Primary Hydra X < ,, , _, 



JJ " " I Medusa X < Eggs. 



X C Medusa X <C Eggs. 



Hydra X ^ Medusa X < Eggs. 



jt 7 ^ <> Medusa X < Eggs. 



Ili/dra X < ,, , j: „ 



J X Medusa X < Eggs. 



In Turritopsis we have essentially the same life-history, except that there is a second- 

 ary alternation between the primary hydra and the others. The planula does not be- 

 come a hydra, but a mouthless, untentaculated root which is undoubtedly a degraded 

 actinula or primary hydra. It does not give rise to medusa buds, but remains as a brood- 

 stock or embryonic hydra from which fully developed hydras are formed by budding, and 

 all of these produce medusa-buds, so the life-history is as follows: 



f Hydra x $ Medusa X < Eggs. 



( Medusa X <C Eggs. 



tt i w( Medusa X <T Eggs. 



VII. Turritopsis Egg = Planula = Moot X j R V dra X \ MeduM x < E ggs. 



Hydra X 5 Medusa X < Eggs. 

 I Medusa X < Eggs. 



In the ordinary Campanularians, with free Medusa?, we have a new element of com- 

 plexity, owing to the appearance of polymorphism. The ordinary hydras no longer give 

 rise to medusa-buds, and these are produced only on the reproductive hydras or blasto- 

 styles. In Eutima, which 1 shall take as an example of this group, we have another com- 

 plication, which is very significant. As in Turritopsis, there is a secondary alternation 

 of generations, for as 1 have shown above, PI. 36, fig. 9, 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. 



