CCELENTERATA : HYDROZOA. 



103 



productive zooid now swims freely by the contractions of its 

 umbrella, and it eats voraciously and increases largely in size. 

 The essential elements of generation are then developed in 

 special cavities in the um- 

 brella, and the fertilised 

 ova, when liberated, ap- 

 pear as free - swimming, 

 ciliated "planulae," which 

 fix themselves, become 

 Hydra - tubce, and com- 

 mence again the cycle of 

 phenomena which we have 

 above described. 



As regards the size of 

 these reproductive zooids 

 as compared with the or- 

 ganism 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 



Fig. 24. Hidden-eyed Medusae. Generative 

 zooid of one of the Pelagidce (Chrysaora 



hysoscella). After Gosse. 



not being more than half 

 an inch in height. 



As regards the special structure of these gigantic reproduc- 

 tive bodies, considerable differences obtain between the Rhizo- 

 stomidce and that section of the Pelagida in which this method 

 of reproduction is employed. In the Pelagida namely, the 

 generative zooids possess a general, though chiefly mimetic, 

 resemblance both to the genuine Discophora and to the free- 

 swimming medusiform gonophores of so many of the Hydrozoa, 

 and they have the following structure. Each (fig. 24) consists 

 of a bell-shaped, gelatinous disc, the " umbrella," from the roof 

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

 extended 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 



