HTDROZOA. 125 



order. They may, without difficulty, be observed 

 in the common species of Lucernaria. They are 

 usually solid, perhaps through vacuolation ; contain 

 thread-cells, and, even when detached, execute a 

 peculiar writhing kind of movement. In Chrysaora, 

 according to Fritz Miiller, they attain a length of 

 several inches. 



Three families of Luceimaridce may be defined 

 as follows : — 



Order LUCERNAEID^. 



Family i. Luceenaeiad^. 



Reproductive elements developed in the 

 primitive hydrosoma, without intervention of 

 free zooids. 



Umbrella with short marginal tentacles 

 and a proximal hydrorhiza. 



Polypite single. 



Family 2. Pelagid^. 



Reproductive elements developed in a free 

 umbrella, which either constitutes the primi- 

 tive hydrosoma or is produced b}'' fission from 

 an attached Lucernaroid. 



Umbrella with marginal tentacles. 



Polypite single. 



Family 3. Rhizostomid^. 



Reproductive elements developed in free 

 zooids produced by fission from attached 

 Lucernaroid s. 



Umbrella without marginal tentacles. 



Poly piles numerous, modified, forming with 

 the genitalia a dendriform mass depending 

 from the umbrella. 



