92 MANUAL OF ZOOLOGY. 



are often of great length. Besides the tentacles, the margin 

 of the umbrella is furnished with a series of peculiar bodies, 

 termed ' lithocysts,' each of which is protected by a sort of 

 process or hood derived from the ectoderm, and consists 

 essentially of a combined 'vesicle' and 'pigment- spot,' such 

 as have been described as occurring in the Medusidce. These 

 marginal bodies likewise communicate with the chylaqueous 

 canals. The reproductive elements ' are lodged in saccular 

 processes of the lower portion of the central cavity, imme- 

 diately above the bases of the radiating canals, and, being 



Pig. 23. Hhizostomidae. Generative zob'id of Rhizostoma. (After Owen.) a. 

 Umbrella, b b. ' Stomatodendra,' covered with clavate tentacles and 

 minute polypites. c c. Anastomosing network of canals. 



usually of some bright colour, form a conspicuous cross 

 shining through the thickness of the disc.' (Greene.) 



In the Rhizostomidce the reproductive zooids differ from 

 those we have just described as occurring in the first section 

 of the Pelagidce, in not possessing tentacles on the margin of 

 the umbrella, and in having the simple central polypite 

 replaced by a composite dendriform process, which bears 

 numerous polypites, projects far below the umbrella, and is 

 thus described by Professor Huxley : ' In the Rhizostomidce 

 (fig. 22) a complex, tree-like mass, whose branches, the 

 " stomatodendra," end in, and are covered by, minute poly- 



