CASSIOPP:A XAMACHANA. 221 



the supporting membrane has disappeared, and the inner boundary of the ectoderm is 

 indistinct. The axial mass of this part of the tentacle is made up of loose particles of a 

 finely granular substance, in which may be seen many small and deeply stained nuclei. 

 There are also a number of green cells that apparently escaped into the central mass 

 when the supporting membrane broke down. There is evidently a free communication 

 between this mass of disintegrating material and the digestive cavity, through the 

 rhopalial canal. 



The method by which the shortening of the tentacle is brought about would seem to 

 be as follows : The axial cells adjoining the cells that bear the concretions (Fig. 54) first 

 break down. Why they should do so, and at this particular time, I cannot say. This 

 disintegration proceeds centrifugally, and it is accompanied by a dissolution of the sup- 

 porting membrane. The ectodermal cells then either begin here and there to break 

 down while still in place, and the resulting debris is squeezed into the central cavity; or 

 else, the cells migrate, or are squeezed inward and then disintegrate. The continuity of 

 the remaining ectoderm is maintained, however. The products of degeneration prob- 

 ably pass through the rhopalial canal into the digestive tract. As this process continues, 

 the inward movement of the ectodermal cells is more rapid than their disintegration, so 

 that when the distal part of the tentacle is reduced to the size of the rhopalial part (Figs. 

 25 and 55), it is a solid mass of small cells with small nuclei that stain dark. Some of 

 these cells contain a large vacuole and have the nucleus pushed to one side. Scattered 

 among the small cells, there are a number of globular bodies as large as, or larger than, 

 the green cells, and completely filled with coarse granules that stain deeply witli safranin; 

 no nucleus is visible in them. The ocellus has now become distinctly cup-shaped (oc. 

 Fig. 55). 



At about this time the interrhopalial tentacles begin to be absorbed in their turn 

 (Fig. 25) . The umbrellar margin has in the mean time grown out beyond the insertion 

 of each interrhopalial tentacle, on its aboral side, into two lobes with a hood between 

 (Figs. 25 and 26). This structure, although smaller, corresponds exactly to the rhopalial 

 lobes and hood, and is further evidence for the homology between the tentacles and the 

 rhopalia. The drawing reproduced in Fig. 25 was made between the hours of eleven 

 in the morning and two in the afternoon. At five o'clock of the same day the tentacles 

 had been reduced to one third the length shown in the figure, and the absorption of the 

 rhopalial tentacles was very nearly completed. 



In the later stages of the absorption of the interrhopalial tentacles, the broken-down 

 material is evidently forced in some way into the radial canal. The rhopalium (Fig. 56) 

 is practically complete at this stage. The point (x) where the last trace of the tentacle 

 proper disappeared, is still distinguishable in sections by the presence of small cells with 

 indistinct cell walls, and by the absence of otoliths. 



