CCELENTERATA : LUCERNARIDA. 



141 



liberated, appear as free-swimming, ciliated "planulae," which 

 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 reproduc- 

 tive bodies, considerable differences obtain between the Rhizo- 

 stomida 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, 

 resemblance both to the genuine Medusidce and to the free- 

 swimming medusiform gonophores of so many of the Hydrozoa, 

 and they have the following structure. Each (fig. 59) consists 

 of a bell shaped, gelatin- 

 ous disc, the "umbrella," 

 from the roof of which is 

 suspended a large poly- 

 pite, the lips of which are 

 extended into lobed pro- 

 cesses often of considera- 

 ble length, "the folds of 

 which serve as temporary 

 receptacles for the ova in 

 the earlier stages of their 

 development." The poly- 

 pite manubrium or pro- 

 boscis is hollowed into 

 a digestive sac, which 

 communicates with a cav- 

 ity in the roof of the um- 

 brella, from which arises 

 a series of radiating ca- 

 nals, the so-called "chyl- 

 aqueous canals." These 

 canals, which are never 

 less than eight in number, 

 branch freely and anasto- 

 mose as they pass towards 

 the periphery of the umbrella, while the entire series is con- 

 nected by a circular marginal canal. This, in turn, sends 

 tubular processes into the marginal tentacles, which are 



Fig. 59. Hidden-eyed Medusae. Generative 

 zooid of one of the Pelagidce (Chrysaora 

 hyoscella), after Gosse. 



