226 ROBERT PAYNE 15KJELOW ON 



margin. The deepest folds are the most distal, and they become progressively more 

 shallow toward the base of the arm. The central mouth is still widely open. The subgeni- 

 tal cavities are well developed at this stage. Figs. 64, 65, and 66 show how the oral disc 

 is formed, and how the subgcnitsil cavities are produced by the great increase in thickness 

 of the mesogloea at the pillars of the proboscis and the bases of the oral arms. By the 

 growth of these structures, the subgenital cavities are necessarily produced. The only 

 special adaptations are the subsequent growth and folding of the aboral wall and the 

 narrowing of the orifice. 



The marginal lobes of the umbrella now begin to broaden, and thus approach the 

 adult condition, but there is only a single "vellar" lobe between two rhopalial ones. 



At a little later stage, when there are three oral funnels at the tips of the arms 

 (Fig. 33), the re-entrant angles between the pillars of the proboscis have grown inward, 

 met at the centre, and fused. In this Avay the lumen of the oesophagus is divided 

 into four tubes (Fig. 32) , representing the grooves that were present in its angles in the 

 earlier stages. In the figure the fusion at the centre has gone so far as to involve the 

 edges of the lips, and the labial grooves of the different pairs of arms are not in open 

 communication, but a short cross-shaped tube connects them at the centre, and the oral 

 disc is now completed. 



It is interesting to note that Clans has found a stage both in Pilema and in Coty- 







lorhiza that, while showing the characteristic family differences, has also a certain 

 resemblance to this stage in Cassiopea. In all three the walls of the proboscis have fused 

 so as to divide its lumen into four tubes, and the formation of oscula has begun at the tips 

 of the arms in such a way that we have on each arm three oscula with a vesicle in the 

 angles between them. The occurrence of this stage in the ontogeny of three so distinctly 

 separated families must have some morphological significance, and we may regard these 

 eight primary vesicles as homologous in the three groups. 



The mode of formation of the oral funnels becomes evident at this stage. They are 

 not formed in Cassiopea simply by a series of fusions of the lips along the line of the 

 labial groove, as Hamann ('8l) states to be the case in Cotylorhiza. It is more like the 

 process in Pilema, as described by Clans. Each of the primary funnels is represented at 

 first by one of the folds in the margin of the lips referred to above (Fig. 33) . The 

 fold deepens, and its edges are brought together on the ventral side and fuse, leaving an 

 opening at the apex of the fold, the osculum. At the same time the labial groove in 

 this region is converted into a canal by the fusion of the lips on its two sides. After 

 the fusion all trace of what has occurred quickly disappears. 



Witli, the division of the oesophagus into four tubes, and the completion of the oral 

 disc, our larva comes to be distinctly a rhizostomatous medusa. Further development of 



