

CCELENTERATA : HYDEOZOA. 91 



originally surrounded the margin of the Hydra-tuba now dis- 

 appears, and a new circlet is developed below the ampliations, 

 at a point a little above the fixed extremity of the Strobila (e). 

 ' The disc-like segments above the tentacles gradually fall off", 

 and, swimming freely by the contractions of the lobed margin 

 which each presents, they have been described by Eschscholtz 

 as true Medusidce under the name of Ephyrce' (/). Each 

 Ephyra, however, soon shows its true nature by becoming 

 developed into a free-swimming reproductive body, usually of 

 large size, with umbrella, hooded lithocysts and tentacles, 

 constituting, in fact, a SteganopWialmate Medusa. The re- 

 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 umbrella, and the fertilised ova, when 

 liberated, appear as free- swimming, ciliated ' planulas,' which 

 fix themselves, become Hydra-tubce, 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 

 mentioned 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- 

 stomidce, 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 Discopliora and to the free- 

 swimming medusiform gonophores of so many of the Eyd/rozoa, 

 and they have the following structure. Each 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 

 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 



