26 Mr. W. K. Brooks on the Origin of 



almost as soon as the buds themselves appear, and it would 

 be difficult to recognize a hydi-a-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 

 hydra is also greatly modified as an adaptation 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 : — 



!Medusa<: Eggs. 

 Medusa '<^Eggs. 

 Medusa'^Eggs. 



1 know of no hydra which presents this life- history without 

 modification ; but there are many Campanularians and Tubu- 

 larians in wliich the only modification is the acquisition by 

 the actinula or primary hydra of the power to produce, in 

 addition to the buds which become raedusa3, 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 Perigomjmus {Stomatoca) the egg gives rise to a 

 planula, which becomes the first hydra, and this produces 

 other hydras like itself, and builds up a liydroid cormus ; and 

 ultimately all these hydras give rise to buds which become 

 directly converted into medusae, the hydra-like stage being 

 completely suppressed ; and we have a life-history like this : — 



rr 7 v^ i Medusa^:rEf/{/s. 



Hydra X ) Medusa^Eggs. 



Hydra x {Mthm^E^gs. 

 I Medusa<zEggs. 

 VI. Egg = Planula = Actinula or Priviary HydruX I 



\ Medusw^Eggs. 



Hydra X ] ^f^f '««<^W*^ 



^ \ I\Jedusa<:Eggs. 



Hvdra X \^Iedusa^Eggs. 



^yil'^a X \Medusa'^Eggs. 



In Turritopsis we have essentially the same life-history, 

 except that there is a secondary alternation between the 

 primary hydra and the others. The planula does not become 

 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 : — 



