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PROCEEDINGS OF THE 



[April, 1857. 



ground where it takes the hydroid form, and at other times it re- 

 mains within the bell-cavity of the parent until it has attained a 

 digestive cavity and tentacula, when it escapes, and moving for 

 awhile by means of these organs, at last becomes fixed like the 

 ciliated forms. In two known cases the hydra-form never becomes 

 fixed, and similar independence is probably realized by a third. 



The embryologicai history of most of these Medusae, so far as 

 known, is one of Alternate Generation, or Metamorphosis, where 

 two of the stages are represented by two distinct individuals. 



There are, however, three stages : first, the Planule, next, the 

 Hydra, and thirdly the Medusa, bearing the sexual organs. 



In two instances, that of tfteginopsis Mediterranean observed by 

 Miiller, Kolliker, and Gegenbaur, and the similar mode of devel- 

 opment observed by myself in the bell-cavity of an Oceania, the 

 hydroid and medusoid stages are not divided between two individ- 

 uals, but the individual Hydra is wholly metamorphosed into the 

 Medusa, thus forming a direct metamorphosis. In both modes 

 there is no intermediate type of form between the hydra and the 

 ultimate medusa stage. 



These Medusae are distinguished from the Discophores proper 

 either by the entire absence of ocelli, or by having them unpro- 

 tected (Gymnopthalmata, Forbes.) They also either entirely want 

 the vail or have it as an unvascular organ which as a perforated 

 septum partially closes the mouth of the bell-like disk. But the 

 most constant difference lies in their embryology for the larva of 

 the Hydroid Medusa passes directly from its polyp form into that 

 of the Hydroid Medusa, while the researches of Steenstrup and 

 Frantzius show that there is a stage in Cyanea and Cephea wherein 

 the form of the Hydroid Medusa is assumed before the animal 

 attains its final form as a Discophore. 



With regard to the subdivisions which may be made in this 

 order, it may be remarked, as Prof. Agassiz has already done in 

 his beautiful monograph, that the organs of essential structure 

 constitute so nearly the totality of a Gymnopthalmatous Medusa, 

 that it is difficult to obtain characters founded on those minor 

 structural details which are usually employed to distinguish 

 genera and families. And Prof. Agassiz in that work expresses 

 the idea that the whole of these animals constitute a single natural 

 family. But when we come to include, as he has done, both the 

 Siphonophorae, the Hydroids proper and the iEginidae in the one 

 order, it appears to me scarcely possible that the varieties of form 



