424 W. K. BROOKS ON THE LIFE-IIISTORY 



itself has an alternation of generations, and the Eleutheria stage does not occur at the 

 beginning of the life-history, but at the end. The Cladonemadse are simply ordinary 

 Anthomedusae, with a tubularian hydra-stage and medusae produced by budding; and 

 it may be due to the unfortunate selection of Eleutheria as an illustration, that Bohm's 

 view, the correctness of which seems to me to be proved by the occurrence of a locomo- 

 tor solitary larval hydra stage in the Trachomedusse, has attracted so little attention. 



His contribution to the subject is a decided advance beyond the views which we have 

 noted, for he shows clearly that we are not compelled to choose between the two alterna- 

 tives which seemed to the other writers to be the only ones, but that the third, viz., that 

 the primitive form was not a sessile community but a locomotor person, has much to 

 commend it; and that we are not compelled to believe with Kleinenberg (75, p. 33) that 

 the alternation is primitive, but that it may have been gradually and secondarily acquired 

 (9, p. 159). Having thus cut himself partially loose from tradition, so far as the antiq- 

 uity of the nurse stage in the history of the. medusa? is concerned, he rests satisfied with 

 the old explanation in other particulars, and states, on p. 159, his belief that the diver- 

 gence from his stem-form, which resulted in the production of the sessile hydra and the 

 locomotor medusa, was brought about by division of labor and polymorphism ; he further 

 says that the locomotor stem-forms multiplied asexually, and thus gave rise to cormi, 

 and that some of the persons of the cormus then became gradually specialized for nu- 

 trition, and were thus converted into sessile hydras, while other persons became special- 

 ized for reproduction, and were gradually converted into true medusae. 



Bohm's paper was published in 1878, and in the same year Claus (15) also showed 

 that the hydra and the medusa are modifications of a common type, of which the actin- 

 ula is the living representative (p. 50), and in his Grundzuge der Zoologie, 1880, he 

 advances an explanation of the origin of the alternation between the hydra generation 

 and the medusa generation, which, so far as I am aware, had never before received at- 

 tention, although there can, I think, be no doubt that it is the true one. 



He discusses the subject very briefly and simply says, on p. G2, that alternation of 

 generations may be between two stages with similar organization, or it may be between 

 the larva ami tin adult as in the JfrJusce. He goes on to point out that Ave must there- 

 fore recognize two distinct kinds of alternation which have originated genetically in two 

 different ways, and have different explanations. We must believe that the second sort 

 of metagenesis, that which resembles metamorphosis, has originated, in most cases, 

 through the retention by the larva of the power to multiply asexually at a stage of de- 

 velopment which, while it may be more or less subject to secondary modification, cor- 

 responds to a remote ancestral stage in the evolution of the species; and that a larva-like 

 nurse stands in the same genetic relation as the larva itself, but the original stem-form, 

 which is now represented by the larva, had in addition to the power of sexual reproduc- 

 tion, which is now restricted to the highly modified adult, the power of asexual multiplica- 

 tion by budding, which has been preserved by the larva in the course of the phylogenetic 

 evolution of the species. On pp. 245-246 he also calls attention to the fact that the 

 actinula larva of Tubularia is a modern representative of the ancestral stem-form, and 

 Tetrapteron oolitans a locomotor ciliated larva which is neither ahydroid nor a medusa, 

 but an indifferent type which might be modified in either direction. 



