io8 



MANUAL OF ZOOLOGY. 



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 Rhizostomidcs 

 and that section of the 

 Pelagidce in which this 

 method of reproduction is 

 employed. In the Pelagidce, 

 namely, the generative 

 zooids possess a general, 

 though chiefly mimetic, re- 

 semblance both to the 

 genuine Discophora and to 

 the free-swimming medusi- 

 form gonophores of so 

 many of the Hydrozoa, and 

 they have the following 

 structure. Each (fig. 25) 

 consists of a bell-shaped, 

 gelatinous disc, the "um- 

 brella," from the roof of 

 which is suspended a large polypite, the lips of which are ex- 

 tended into lobed processes often of considerable length, " the 

 folds of which serve as temporary receptacles for the ova in 

 the earlier stages of their development." 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 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 



Fig. 25. — Hidden-eyed Medusae. Generative 

 zo5id of one of the Felagidte {Chrysaora 

 kysoscella), after Gosse. 



