I 1 6 MANUAL OF ZOOLOGY. 



the two, except as concerns the structure of the generative 

 zooids. 



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

 are defined as being Lucernarida in which the reproductive 

 elements are developed in free zooids, 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. -33), the illustration, 

 however, representing the development of Aurelia, one of the 

 Pelagida, in which the phenomena are essentially the same. 

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

 termed a "planula" (d), 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 communi- 

 cates 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 at this stage of 

 its career has been termed by Sir J. G. Dalyell. The mouth, 

 in the meantime, from being a .mere quadrilateral orifice, 

 grows and lengthens itself so as to constitute 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 resemble 

 what may be seen in Lucernaria" (Greene, Manual of 

 Ccelenteratd). The Hydra-tuba thus constitutes the fixed 

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

 midce. 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. 33). The 

 Hydra-tuba becomes elongated, and becomes marked by a 



