106 MANUAL OF ZOOLOGY. 



Order III. Rhizostomid/E. — The members of this order 

 are defined as being Lucemarida, in which the reproductive 

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

 tached Lucernaroids. The umbrella of the generative zooids is 

 ■without marginal tentacles, and the polypites are " numerous, 

 fnodified, 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. 24), the illustration, 

 however, representing the development of Chrysaora, one of the 

 ' Pelagidce, 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 ; otheri 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-ttcbd, closely re- 

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

 of Cmlenterata.) The Hydra-tuba thus constitutes the fixed 

 " Lucemaroid," or the " trophosome " of one of the Rhizcsto- 

 midm. In height it is less than half an inch, but it possesses 

 the power of forming, by gemmation, large colonies, which may 

 remain in this condition for years, the organism itself being 

 incapable of producing the essential elements of generation. 

 Under certain circumstances, however, reproductive zooids are 

 produced by the following singular process (fig. 24).- The 

 Hydra-tuba becomes elongated, and becomes marked by a se- 

 ries of grooves or circular indentations, extending transversely 

 across the body from a little below the tentacles to a little above 

 the fixed extremity. At this stage the organism was described 



