CCELENTERATA : HYDROZOA. : fO 



thereby differing from Lucernaria, which it approaches, on the 

 other hand, in the fact that its generative elements are pro- 

 duced in its own umbrella without the intervention of free 

 generative zooids. Pelagia, however, differs considerably in 

 structure from Lucernaria, and in all essential characters is not 

 anatomically separable from a Steganophthalmate Medusid. The 

 process of reproduction as displayed in the first section of the 

 Pelagidcz will be considered when treating of that of the Rhi- 

 zostomidcz, there being no important difference between the 

 two, except as concerns the structure of the generative zooids. 



ORDER III. RHIZOSTOMID^:. The members of this order 

 are denned as being Lucernarida, in which the reproductive 

 elements are developed in free zooids, produced by fission from at- 

 tached Lucernaroids. The umbrella of the generative zooids is 

 without marginal tentacles, and the polypites are " numerous, 

 modified, forming with the genitalia a dendriform mass depending 

 from the umbrella" (Greene.) 



The following is a brief summary of the life-history of a 

 member of this extraordinary order (fig. 23), the illustration, 

 however, representing the development of Chrysaora one of the 

 Pelagidcz, in which the phenomena are essentially the same. The 

 embryo is a free-swimming, oblong, ciliated body, termed a 

 " planula " (a), of a very minute size, and composed of an outer 

 and inner layer enclosing a central cavity. The planula soon 

 becomes pear-shaped, and a depression is formed at its larger 

 end. " Next, the narrower end attaches itself to some sub- 

 marine body, whilst the depression at the opposite extremity, 

 becoming deeper and deeper, at length communicates with the 

 interior cavity. Thus, a mouth is formed, around which may 

 be seen four small protuberances, the rudiments of tentacula. 

 In the interspaces of these four new tentacles arise ; others in 

 quick succession make their appearance, until a circlet of nu- 

 merous filiform appendages, containing thread-cells, surrounds 

 the distal margin of the ' Hydra-tuba ' (b), as the young organ- 

 ism at this stage of its career has been termed by Sir J. G. 

 Dalyell. The mouth, in the mean time, from being a mere 

 quadrilateral orifice, grows and lengthens itself so as to consti- 

 tute a true polypite, occupying the axis of the inverted umbrella, 

 or disc, which supports the marginal tentacles. The space 

 between the walls of the polypite and umbrella is divided into 

 longitudinal canals, whose relations to the rest of the organism, 

 and, indeed, the whole structure of Hydra-tuba, closely re- 

 semble what may be seen in Lucernaria." (Greene, Manual 

 of Cczlenterata.} The Hyra-tuba thus constitutes the fixed 

 " Lucernaroid," or the " trophosome " of one of the Rhizosto- 



