XIV. The Life-History of the IIydromedusje : 



A Discussion of the Origin of the Medusae, and of the 

 Significance of Metagenesis. 



By "W. K. Brooks, Johns Hopkins University, Baltimore, Md. 



(Read Oct. 7, 1885.) 



IVlOST recent writers upon the origin of the sexual Medusa- which arc set free 

 from communities of sessile hydroids, and upon the relation between them and the 

 hydroids, agree in the opinion that the sessile community is the primitive form from 

 which the medusae have been derived, and that the medusae have originated through the 

 gradual specialization of the reproductive members of a polymorphic hydroid-cormus. 



This opinion is . generally, but not universally, accepted for Bohm (9) has given 

 his reasons for believing that the medusae have arisen from floating - , rather than fixed 

 hydroids ; and Claus has advanced the opinion that the medusa is older than the poly- 

 morphic hydroid-cormus, that the hydra is simply a medusa-larva, and that the alter- 

 nation of generations has originated through the power to multiply asexually which this 

 larva possesses ; and that the alternation of generations is therefore a secondary modifi- 

 cation of a life-history which was originally simple and direct. 



Neither of these writers refers to the life-history of the Narcomedusae and Tracho- 

 medusae, and the purpose of the present paper is to show that the metamorphosis of 

 these medusse furnishes direct disproof of the polymorphism-hypothesis, and completely 

 establishes the explanation advanced by Bohm and Claus, through evidence which neither 

 of these authors discusses. 



I may also be allowed to state that I was led, several years ago, by the study of the 

 development of the Trachomedusse and Narcomedusa\ to the conclusions which are here 

 given, before I was aware that Bohm and Claus had also arrived at the same view of 

 the relation between the medusa and the hydra. As this is my first opportunity 

 to publish the illustrations which are necessary for demonstrating the correctness of this 

 conclusion, I now select, from notes which I have made on the Medusa? of Beaufort 

 during the past six years at the Marine Laboratory of the Johns Hopkins University, 

 those observations which are best adapted for illustrating my view of the origin and 

 significance of alternation of generations or metagenesis in the Hydromedusae. This 

 paper therefore contains an account of the life-history of a Narcomedusa, Cunocantha 



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