n8 



MANUAL OF ZOOLOGY. 



fix themselves, become Hydra-tuba, and commence again the 

 cycle of phenomena which we have above described. 



As regards the size of these reproductive zooids as compared 

 with the organism by which they are given off, it may be men- 

 tioned that the umbrella of Cyanea arctica -has been found in 

 one specimen to be seven feet in diameter, with tentacles more 

 than fifty feet in length, the fixed Lucernaroid from which it 



was produced not being 

 more than half an inch in 

 height. 



As regards the special 

 structure of these gigantic 

 reproductive bodies, con- 

 siderable differences obtain 

 between the Rhizostomi- 

 dce and that section of the 

 Pelagidce in which this 

 method of reproduction is 

 employed. In the Pelagi- 

 dce, namely, the generative 

 zooids possess a general, 

 though chiefly mimetic, re- 

 semblance both to the 

 genuine Medusidce and to 

 the free-swimming medu- 

 siform gonophores of so 

 many of the Hydrozoa, and 

 they have the following 



Fig. 34.-Hidden-eyed Medusa,. Generative Structure. Each (fig. 34) 

 zooid of one of the Pelagidce (Chrysaora COnSlStS of a bell-Shaped 

 hysoscelld), after Gosse. -, . , , ,/ 



gelatinous disc, the " um- 

 brella," from the roof of which is suspended a large polypite. 

 the lips of which are extended into lobed pK>cesses often of 

 considerable length, "the folds of which serve as temporary 

 receptacles for the ova in the earlier stages of their develop- 

 ment." The polypite manubrium or proboscis is hollowed 

 into a digestive sac, which communicates with a cavity in the 

 roof of the umbrella, from which arise a series of radiating 

 canals, the so-called "chylaqueous canals." These canals, 

 which are never less 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 processes into the marginal 

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

 the margin of the umbrella is furnished with a series of peculiar 



