CCELENTERATA : LUCERNARIDA. 



139 



Rhizostomidce, there being no important difference between 

 the two> except as concerns the structure of the generative 

 zooids. 



ORDER III. RHIZOSTOMIDCE. 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 zoo'ids 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. 58), the illustration, 

 however, representing the development of Aurelia^ one of the 



Fig. 58. Development of Aurefia, one of the Lucernarida. a Ciliated free-swimming 

 embryo, or " planula ; " b Hydra-tuba; c Hydra-tuba in which fission has consider- 

 ably advanced, and the " Strobila" stage has been reached ; d Hydra-tuba in which 

 the fission has proceeded still further, and a large number of the segments have been 

 already detached fo lead an independent existence. 



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 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 



