MA'l'EIilALS FOB A MONOGRAPH OF THE ASCONS. 481 



nuclei of the flat epithelium and those of the collar-cells ; 

 speaking generally, the diameter of the latter is about four 

 fifths of that of the former (compare PI. 41, figs. 39 and 40), 



The cytoplasm of the epithelial cells is packed with granules 

 of a very characteristic kind, which easily distinguish the flat- 

 tened epithelium and its derivatives, the pore-cells and the 

 spicule-cells, from either the collar-cells or the wandering cells. 

 As a convenient type in which to study the granulation of the 

 epithelial cells, we may take Clathrina coriacea (figs. 1 — 

 16). Seen in osmic-picrocarmine-glycerine preparations these 

 granules appear of rounded but rather irregular outline, and 

 vary both in size and depth of colour. Some are pale, while 

 others appear darker and even quite black. If one of the 

 paler granules be carefully foCussed, it appears to have a dark 

 border surrounding a clear central spot. On slightly raising 

 the focus the clear central spot disappears, and at a still 

 higher focus a dark central spot is seen surrounded by a clear 

 area. The darker granules, of which there are usually several 

 in each cell, do not show these changes distinctly, but appear 

 simply black. In this species the granules have, in prepara- 

 tions of the kind mentioned, a yellowish or yellowish-brown 

 shimmer, and impart a similar tint to the protoplasm of the 

 whole cell. In general the granules appear opaque and dull, — 

 in short, they may be described as having an almost chitinous 

 appearance. In life they are the elements to which the sponge 

 owes its colour, whatever it may be. If the sponge is white, 

 which is more usual, the granules appear black in transmitted 

 light, dull white in reflected light. In the numerous colour 

 varieties of C. coriacea the granules appear red, orange, 

 yellow, lilac, or whatever may be the colour of the sponge, in 

 reflected light; and in Clathrina elathrus they similarly 

 have a constant lemon-yellow colour. 



The distribution of the granules varies slightly with the 

 condition of the cell, though they are always more or less con- 

 centrated round the nucleus. When the cell is fully expanded 

 the nucleus is superficial, with very few granules or none at 

 all over it, that is to say external to it, and the granules are 



