IO4 



MANUAL OF ZOOLOGY. 



than eight in number, branch freely and anastomose as they 

 pass towards the periphery of the umbrella, where the entire 

 series is connected by a circular marginal canal. This, in 



turn, sends tubular pro- 

 cesses into the marginal 

 tentacles, which are often 

 of great length. Besides 

 the tentacles, the margin 

 of the umbrella" is fur- 

 nished with a series of 

 peculiar bodies, termed 

 " lithocysts," each of 

 which is protected by a 

 sort of process or hood 

 derived from the ecto- 

 derm, and consists es- 

 sentially 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 



Fig. as.-Rhizostomidae. Generative zooid of reproductive elements 

 Rhizostoma. (After Owen.) a Umbrella ; b b " ar lded i 



are lodged in 



" Stomatodendra," covered with clavate ten- r , , 



tacles and minute polypites ; c c Anastomosing prOCCSSCS Oi trie iOWCr 



network of canals. portion of the central 



cavity, immediately 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. 25) differ 

 from those we have just described as occurring in the first 

 section of the Pelagidtz, 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 Rhizo- 

 stomidce (fig. 25) a complex, tree-like mass, whose branches, the 

 1 Stomatodendra,' end in, and are covered by, minute polypites, 

 interspersed with clavate tentacula, is suspended from the middle 

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

 dependent polypiferous tree, in fact, unite above into a thick, 

 flat, quadrate disc, the ' syndendrium,' which is suspended by 

 four stout pillars, the ' dendrostyles/ one springing from each 



