CGELENTERATA I HYDROZOA. I i 9 



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 Medusida. '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- 



Fig. 35- Rhizostomidae. Generative zooid of Rhizostoma (after Owen). a Um- 

 brella ; b b " Stomatodendra," covered with clavate tentacles and minute polypites ; 

 c c Anastomosing network of canals. 



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

 usually of some bright colour, form a conspicuous cross shining 

 through the thickness of the disc" (Greene). 



In the Rhizostomidcz the reproductive zooids (fig. 35), differ 

 from those we have just described as occurring in the first 

 section of the Pelagidcz, 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 Rhizostomida 

 (fig- 35)5 a complex, tree-like mass, whose branches, the ' stoma- 

 todendra,' end in, and are covered by, minute polypites, inter- 

 spersed with clavate tentacula, is suspended from the middle 

 of the umbrella in a very singular way. The main trunks of 



