130 



TRACHYMEDUS^E. 



The lining of the cavity so formed is derived from the epiblast of the 

 disc already spoken of. 



The exact mode of formation of the gastrovascular canals has not been 



worked out. It has however been 

 established by the researches of the 

 Hertwigs (No. 146) and Glaus (No. 

 153) that the radial and circular 

 vessels of this system are connected 

 together in adult Medusae by an 

 hypoblastic lamella ; so that these 

 canals would seem to be the rem- 

 nants of an once continuous gastric 

 cavity. This mode of formation is 

 established in the case of the medu- 

 siform buds ; and it would therefore 

 seem, as pointed out by the Hert- 



wigs, a fair deduction that it occurs 



FIG. 72. OPTICAL SECTION THROUGH THE 

 ORAL POLE OF GfiRYONIA AFTER THE APPEAR- 

 ANCE OF THE GELATINOUS TISSUE OF THE DISC. 

 (After Fol.) 



o. mouth ; v. velum ; t. tentacle. 



The shaded part represents the gelati- 

 nous tissue. 



in the larva a conclusion which is 



confirmed by the primitive extension of the gastric cavity to the edge of 

 the disc at the time when its walls give rise to the solid axes of the tentacles. 

 In the course of the subsequent retirement of the gastric cavity from the 

 edge of the disc the gastrovascular canals probably take their origin, 

 though Fol was unable to follow the changes which result in their 

 formation. 



On the completion of the above changes the larva has become a 

 fully formed Medusa, but it undergoes a not inconsiderable metamor- 

 phosis before the attainment of the adult state. 



Two species of ^Eginidse have been studied by Metschnikoff (163), 

 viz. Polyxenia leucostyla (^Ecjineta flavescens), and JEginopsis mediter- 

 ranea. In both of these forms the segmentation results in the forma- 



Fir,. 73. A THREE-DAYS LARVA OF ^EoiNOPSIS WITH TWO TENTACLES. (After 



Metschnikoff.) 

 m. mouth; t. tentacle. 



tion of an elongated two-layered ciliated planula, without a central 

 cavity. The two ends of this grow out into two long processes the 

 rudiments of a pair of at first aborally directed arms which contain a 



