412 W. K. BROOKS ON THE LIFE-HISTORY 



and its power of locomotion in order 1o secure the diffusion of the reproductive elements. 

 Gregenbaur (26) and Balfour (65) tell him thai the medusa is not an organ but a per- 

 son, homologous with a whole hydroid, not with a part of it as Huxley teaches; that 

 the separation of the community into sessile nutritive hydra-persons and locomotor re- 

 productive medusa-persons lias been brought about by division of labor; that the hy- 

 dra community is older than the medusa; that originally all the members of the com- 

 munity were alike; that gradually certain ones became set apart for reproduction; and 

 that, finally, these latter were set free, and acquiring reproductive organs became loco- 

 motor medusae and that this change was brought about in order to secure the diffusion 

 of the reproductive elements. These authors also believe that after medusas had been 

 gradually evolved in this way for this purpose, circumstances changed in some unex- 

 plained way, so that the wide diffusion of the reproductive elements was no longer so 

 essential and that the medusa? took the back track and retrograded into sessile medusa- 

 bnds. 



Hamann (32) who also believes that the sessile community is the primitive form, and 

 that the medusae have been produced by the gradual specialization of certain members 

 which were set apart as the reproductive members, holds that they gradually acquired 

 the power of locomotion in order to secure cross-fertilization rather than the diffusion of 

 the eggs. 



In 1878 Bohm (9) showed that the opinion that the sessile community is the primi- 

 tive form, presents insuperable difficulties and he points out many reasons for believing 

 that both the fixed hydra and the locomotor medusa have been evolved from a floating 

 actinula, and Clans, two years after (17), 1880, states very briefly his belief that the 

 hydra-stage is a larva and the medusa simply the adult and that the alternation of 

 generations is due to the fact that the larva has the power to multiply asexually and thus 

 to produce a number of larva? like itself. This view is identical with the one which I 

 reached independently at about the same time, before I was acquainted with the conclu- 

 sions of Bohm and Claus, from the evidence which I am now able to present in full with 

 illustrations, and which is not the same as the evidence which led Claus and Bohm to 

 the same result, tor neither of these authors makes any special reference to the life-history 

 of the JSTarcomedusae and Trachomedusae. 



It seems to me that the facts which are given in this paper establish this view beyond 

 controversy and I shall show in the review of the literature of the subject which is given 

 farther on that it is the only hypothesis to which there are not insuperable objections. 

 Even if this were not the case, I think that a comparison of the life-histories which are 

 represented in the nine diagrams given above would convince every one that they stand 

 in some derivative relation to each other, and it is surely simpler to believe that the com- 

 plicated life-history shown in diagram VIII has been derived from a simple one like that 

 shown in diagram I, than it is to believe with Balfour, Hamann and Grobben that the 

 Narcomedusa? and Trachomedusae have been produced as the reproductive members of 

 polymorphic cormi and that they have afterwards lost all traces of this ancestry. 



Most of the reasons which compel us to this conclusion will be brought out in the re- 

 view of the literature of the subject, but I wish to call attention here to one argument 

 which is not noticed elsewhere, although it seems to me to be entitled to great weight. 



