158 TRACHYMEDUS^E. 



Hertwigs (No. 146) and Glaus (No. 153) that the radial and circular vessels 

 of this system are connected together in adult Medusas by an hypoblastic 

 lamella ; so that these canals would seem to be the remnants of an once- 

 continuous gastric cavity. This mode of formation is established in the case 

 of the medusiform buds ; and it would therefore seem, as pointed out by the 

 Hertwigs, a fair deduction that it occurs 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 

 metamorphosis before the attainment of the adult state. 



Two species of ^Eginidae have been studied by MetschnikofT 

 (163), viz. Polyxenia leucostyla (^Egineta flavescens), and JEgi- 

 nopsis medittrranea* In both of these forms the segmentation 



FIG. 73. A THREE-DAYS' LARVA OF ^EGINOPSIS WITH TWO TENTACLES. 



(After MetschnikofT.) 



///. mouth; /. tentacle. 



results in the formation 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 solid hypoblastic axis 

 (fig. 73). At this stage the larva closely resembles the larva of 

 Tubularia. An alimentary cavity is hollowed out in the centre 

 of the hypoblast which soon opens by a wide oral aperture (in). 

 A second pair of arms becomes formed, which are at first much 

 shorter than the original pair; with their formation a radial 

 symmetry is acquired. Sense-organs become at the same time 



