﻿April, 1857.] 



ELLIOTT SOCIETY. 



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the development of Aeginopsis Mediterranea observed by Miiller, 

 Kolliker and Gegenbaur, of Stenogaster complanatus by Kolliker, 

 and by the similar embryological history observed by myself in the 

 bell-cavity of Oceania (Turritopsis) nutricula. In each of them 

 the polyp or hydra-form is not persistent, and Medusae are not pro- 

 duced by gemmation from Hydrae, but the Hydra itself is indi- 

 vidually and totally metamorphosed into the Medusa, thus at once 

 demonstrating that the Medusae cannot be the young of the Hydrae, 

 and that there is no essential difference between the so-called al- 

 ternation of generations and a direct and regular metamorphosis, 

 or homogony. The inevitable conclusion must be that Hydras are 

 either young or inferior forms of Acalephae, and consequently 

 that they are not entitled to independent positions as perfect ani- 

 mals, but must be described as Larvae simply. 



With regard to the difference between the Siphonophorae and 

 Hydroidea, it is greater in appearance than in reality. For the 

 Campanularians, Tubularians and Hydractinians in which the Me- 

 dusae do not become free are just as really communities of hydroid 

 and medusiform individuals as Siphonophorae, differing only in the 

 comparatively unimportant circumstance that they are attached 

 forms, and that in them specialization is not carried so far, so that 

 instead of having from three to five kinds of individuals, we there 

 find only from two to three. 



For the foregoing reasons, I include in this Report on the Hy- 

 droid Medusae (Craspedota Gegenb.) of Charleston Harbor, all the 

 Hydroidea and Siphonophorae, known to me, describing the Hy- 

 droids as larval forms in connection with their perfect forms, 

 where these have been ascertained, and giving them provisional 

 names while their Medusae are still unknown. At the same time 

 I should state that I lay no claim to originality in thus limiting 

 the order. The idea in some form or other has been floating in 

 the minds of those who have particularly studied this subject ever 

 since the appearance of Steenstrup's remarkable treatise on Al- 

 ternate Generation, and Agassiz, Huxley, Leuckhart, Owen, Kolli- 

 ker, and Gegenbaur have all expressed it in some form or other. 

 Prof. Agassiz, in his lectures, has given the group those limits 

 which I now assign it.* 



*The delay which has unavoidably attended the publication of this paper, 

 enables me to state that Prof. Agassiz, in the first of his elegant volumes on the 

 Natural History of America, gives this order the limits which he had formerly 

 given in his lectures, and applies the name of Hydroidea to it, according to the 

 rule of priority. June, 1858. 



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