418 W. K. BROOKS ON THE LIFE-HISTORY 



been produced by the modification of 1, and 4 by the modification of 3, and that the lo- 

 comotor habits of 2 and 4 have been independently acquired. 



In each case we have a pulsating, gelatinous bell, with sub-umbral muscles and a veil; 

 a pendent stomach with ova or spermatozoa developed in its walls; four radial canals, a 

 circular canal and hollow marginal tentacles; and the two medusae are almost as much 

 alike as the hydroids 1 and 2, or the hydroids 3 and 4. The chances are very greatly 

 against the independent modification of the two forms along lines which are so perfectly 

 parallel, and when we bear in mind that the hypothesis compels us to believe that this 

 has taken place not in two but in many cases, the difficulty becomes a very great one; 

 but, if we adopt the opposite hypothesis, and regard the medusa-bud as a degraded, ses- 

 sile medusa, there is no such difficulty, for similar medusae would give rise, by degrada- 

 tion and the loss of their locomotor apparatus, to similar medusa-buds. Then, too, if the 

 medusa-buds are stages in the process which has led to the formation of free medusas, 

 we cannot account for the presence in buds which never became free, of structures which 

 like the bell-cavity and velum are of functional importance only in the swimming 

 medusa?, although we should expect these organs or their rudiments to be retained by 

 medusa? which had lost their swimming habits and become sessile. 



These and other facts have led most naturalists to believe with Koch that the me- 

 dusa? are not specialized reproductive organs, but modified hydras, and that the sessile 

 medusa-buds are degraded medusa? rather than stages in the evolution of medusa?. The 

 life-history of Lirioj)e seems to be totally irreconcilable with Huxley's view, for this 

 would require us to believe that the egg here gives rise to nothing but a reproductive 

 organ and that this process is continued generation after generation. 



I think, therefore, that the facts justify the statement that our present knowledge of 

 the subject disproves the view which Huxley advocates and that this view is now unten- 

 able. As a matter of fact, nearly all naturalists reject it in favor of the " polymorphism " 

 hypothesis, which the student will find presented in the text-books of Gegenbaur and 

 Balfour, but examination of the special literature will show that the various advocates 

 of this hypothesis are by no means agreed as to the precise manner in which the two 

 polymorphic forms, the hydra and the medusa, have been produced. 



Balfour, for example (65), adopting essentially the views which had been brought 

 forward many years before by Leuckart, says, "The chief interest of the occurrence of 

 alternation of generations among the Hydromedusa? and Siphonophora is the fact that 

 its origin can be traced to a division of labor in the colonial system of zooids so charac- 

 teristic of these types. In the Hydromedusa' an interesting series of relations between 

 alternation of generations and the division of the zooids into gonophores and trophosomes 

 can be made out. In Hydra the generative and nutritive functions are united in the 

 same individual. * * * A condition like that of Hydra in which the ovum directly gives 

 rise to a form like its parent is no doubt the primitive one. * * * The relation of Hydra 

 to the Tubularidse and Campanularida? may be best conceived by supposing that in Hy- 

 dra most ordinary buds did not become detached so that a compound hydra became 

 formed, but that at certain periods particular buds retained their primitive capacity of 

 becoming detached and subsequently developed reproductive organs, while the ordinary 

 buds lost their generative function. It would obviously be advantageous to the species 



