STUDIES ON THE COMPARATIVE ANATOMY OP SPONGES. 213 



ectoderm and endoderm somewhere) I follow him in this 

 respect^ and regard both layers of epithelial cells in the oscular 

 diaphragm of Vosmaeropsis Wilsoni as ectodermal also. 

 As there is no structural distinction between the flattened 

 endodermal cells which line the gastral cavity in Heterocoele 

 sponges and the ectodermal cells, it is impossible to say exactly 

 where one begins and the other ends. 



A portion of the epithelium which covers the upper surface 

 of the oscular diaphragm in the sponge in question is repre- 

 sented in fig. 63, drawn from an osmic acid preparation 

 mounted in glycerine. The cells are mostly polygonal, and 

 fit tightly together by their edges. Each has the form of a 

 flat plate, with usually a single nucleus situated in about 

 the centre. The nucleus is small, and in osmic acid pre- 

 parations looks clearer and more transparent than the 

 surrounding protoplasm. The latter contains numerous 

 granules, larger and more abundant towards the centre of 

 the cell, which are stained very darkly by the osmic acid. 

 The majority of the cells are about as broad as they are long, 

 but some of them are fusiform, like those of the Leucosolenia 

 diaphragm figured by Minchin in his fig. 12. On the under 

 surface of the diaphragm a similar layer of cells occurs, but 

 these are not so deeply stained by the osmic acid, doubtless 

 owing to want of penetration. 



I do not doubt that these epithelial cells are contractile, 

 and that their contractility serves to open and close the dia- 

 phragm. 



If we trace this epithelium from the oscular diaphragm up 

 over the lip of the osculum and on to the outer surface of the 

 sponge, we find that it maintains exactly the same character 

 throughout, covering the whole outer surface of the sponge 

 with a layer of flattened cells. The dermal cortex is penetrated 

 by very numerous inhalant pores (fig. 23), and on reaching the 

 margin of one of these the ectodermal epithelium turns in, 

 and is continued as a lining to the inhalant canal system, a 

 lining which resembles minutely the epithelium of the oscular 

 diaphragm, and the component plate-like cells of which are 



