STRUCTURE OF THE LARVA OF SPONGILLA LACUSTRIS. 411 



presents an extraordinary change in the arrangement of the 

 cells. Its constituent cells are migrating into the interior 

 individually, and consequently the layer itself becomes less 

 dense, the cells shorten, and at the same time become broader, 

 owing to the superficial expansion of the sponge as a whole. The 

 above changes in the characters of the flagellated cells might 

 seem at first sight to support the view, held by many authors, 

 that the flagellated cells become the flattened epithelium. 

 But a more careful study of the figures is sufficient to refute 

 any such idea, and to force us to the conclusion that what- 

 ever happens to the flagellated cells, they do not flatten 

 out to become the constituent cells of the dermal epithe- 

 lium. In the first place, the nuclei of the flagellated cells 

 are seen to be passing in, though it is almost impossible 

 to make out the cell body, and the portion nearest the margin 

 of the pupa is full of them (figs. 30 and 30 a). In the second 

 place, the cells with granular nuclei have come to the surface 

 already in the portion nearest the margin (fig. 30), while 

 nearest the centre they are seen engaged in the struggle, so 

 to speak, of passing to the exterior between the few remain- 

 ing flagellated cells (fig. 30«). And in the third place, the 

 nuclei of the cells, which before metamorphosis were situated 

 in the interior, retain their large size and granular character, 

 while those of the flagellated cells, which were once at the 

 surface, are undergoing the usual changes through which they 

 pass when they are about to enter into the composition of the 

 plasmodial aggregations. 



There is, therefore, no reason whatever to suppose that the 

 flagellated cells flatten out and become indistinguishable from 

 the cells with granular nuclei, which they would have to do 

 were they the cells from which the dermal epithelium is 

 developed. 



The flagellated cells have been traced to the interior already, 

 and their nuclei have been shown to be undergoing the changes 

 usual in the formation of plasmodial aggregations. It remains 

 to show what further changes they will pass through. In the 

 pupa which is represented in fig. 31 the flagellated cells are 



VOL. 42, PART 4. NEW SERIES. P p 



