CCELENTEKATA : HYDKOZOA. 89 



duced within the body of Lucernaria itself, without the inter- 

 vention of any generative zooid. 



ORDER II. PELAGID^;. This order is denned as including 

 Lucernarida, which possess a single polypite only and an um- 

 brella with marginal tentacles. The reproductive elements ' are 

 developed in a free umbrella, which either constitutes the primitive 

 hydrosoma, or is produced by fission from an attached Lucerna- 

 ro'id.' (Greene.) 



Two types, therefore, exist in the Pelagidce. The one type 

 is represented by a fixed ' trophosome,' resembling Lucernaria, 

 but distinguished from it by the fact that the generative ele- 

 ments are not developed in the primitive hydrosoma, but in a 

 free 'gonosome,' which is produced for the purpose. The 

 second type, represented by Pelagia itself, is permanently free, 

 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 

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

 zostomidcv, there being no important difference between the 

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



ORDER III. BHIZOSTOMID^E. The members of this order 

 are defined as being Lucernarida, in which the reproductive 

 elements are developed in free zooids t produced by fission from 

 attached 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. 22). 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 submarine 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 numerous 

 filiform appendages, containing thread- cells, surrounds the 

 distal margin of the " Hydra- tuba " (b), as the young organism 



