404: W. K. BROOKS ON THE LIFE-HISTORY 



conies directly converted into a single hydra, as in Tubulariaj where there is no meta- 

 genesis, luit in many other forms there is certainly a true alternation between the planula 

 stage ami the hydra stage. 



The direct conversion of the ciliated, mouthless planula into the tentaculated stoma- 

 tous hydra will, without doubt, he recognized as the primitive life-history; and the alter- 

 nation of generations between the planula, or the root into which it becomes converted, 

 and the hydras formed from it by budding, will, I think, be universally accepted as a 

 secondary modification. I shall give, in the last section of this paper, my reasons for be- 

 lieving that the alternation of generations between the hydra and the medusa is not primi- 

 tive but secondary, and that originally a tentaculated hydra-like larva became directly 

 metamorphosed into a single medusa; and the fact that an alternation of generations be- 

 tween the planula and the hydra has been secondarily established in Hydractinia, Eu- 

 tima and Obelia certainly shows that this view is not without the support of analogy. 



The oldest hydranths of Eutinia which 1 reared from the egg were like the large one 

 in fig. 10. They had ten tentacles, five long ones alternating with five short ones, with 

 their bases united by an intertentacular web, Jc, in the centre of which there is a rounded 

 hemispherical manubrium; ending in a simple circular mouth, without oral tentacles. 



Although the hydroid is Campanularia-like, the perisarc is not annulated and is con- 

 fined to the root and stems and does not extend over the bodies of the hydranths, which 

 therefore belong to Clans' genus Campanopsis (66) from which he has reared a medusa 

 which is very closely related to Eutima, and which belongs to the same genus as origi- 

 nally established by McCrady. The metamorphosis of the young medusa has been well 

 described by Clans for his species and there is no reason to suppose that ours is essen- 

 tially different. 



In the species which he studied, Octorchis Gegeribauri, the medusa-buds originate on 

 the body of the hydranth, on short pedicels, and they are inclosed in mantles or capsules 

 which are cellular and without a covering of perisarc. When set free, the bell of the 

 medusa is deep, the height being somewhat greater than the radius. There is no ped- 

 uncle, and the stomach, which is less than half as long as the depth of the sub-umbrella, 

 ends in a simple mouth without lips. There are two opposite radial tentacles, the rudi- 

 ments of two others, numerous solid marginal cirri, and eight adradial marginal vesicles, 

 each with a single ocellus. Clans did not succeed in keeping the medusa? alive, but he 

 traced in a series of captured specimens the gradual increase in the number of tentacles, 

 the growth of the peduncle and lips and the development of the reproductive orpins, 

 the peduncular portions of which appear earlier than the sub-iunbral portions. 



The Origin of Alternation of Generations in the Hydromeousje. 



In the experimental sciences, the investigator seeks for the simplest manifestations of 

 the natural law which he wishes to study, and divests it, as far as possible, of all second- 

 ary complications. In the natural sciences where experiment is usually impossible, the 

 phenomena must be studied as they present themselves in nature; and the difficulties 



