Sponge- fauna of Noi' way . 151 



It is in these areas, which may be even and plain, or sub- 

 divided by smaller fibrous threads into a number of round 

 or oval spaces, that the pores are situated : they are 

 very small, from 0*001 to 0*002 inch in diameter; and the 

 poriferous membrane is so tender and delicate that it would 

 probably be torn away by an observer unsuspicious of its pre- 

 sence, and in this way may have escaped the notice of Bower- 

 bank ; when removed, the projecting spicular fibres a])pear 

 below, each rising out of a fleshy papilla, the lower half of 

 what we shall term a spicular column of the cortex. A good 

 representation of the surface of the sponge, as thus denuded 

 of its dermal membrane, is given by Johnston {loc cit. pi. i. 

 fig. 3). The spaces between the papilla (PI. VII. fig. 6), 

 roofed over by the dermal membrane in the uninjured sponge, 

 correspond to the intermarginal or subdermal cavities of other 

 sponges. 



Bisecting the sponge through its oscule, we distinguish on 

 the cut face an external whitish rind and a pale greyish mark 

 sharply defined from it ; the skin and subdermal cavities are 

 readily observable, forming the outer half of the rind ; its 

 inner half is a continuous whitish layer. In the mark, nume- 

 rous canals are seen cut across ; and one large one approaching 

 close to the oscule, along a spiral course conformable to that 

 of the spicular fibres, is clearly one of the main excurrent 

 canals. 



We now proceed to give a more detailed account of the 

 structure of the sponge, as revealed in thin slices examined 

 under the microscope. 



The Ectoderm. — The study of this layer is full of perplexing 

 difficulties, owing partly, no doubt, to the fact that one is 

 limited to particular methods in investigating it, but partly 

 also to the want of constancy in those characters which it 

 clearly displays. It is in the subdermal cavities that its 

 structure is most satisfactorily seen. There, on the sides of the 

 spicular columns (PL VII. fig. 13) one may sometimes discover 

 it as a superficial layer of irregularly polygonal plate-like 

 cells, xsinr to ^o u i^^^^ i^ diameter, with small circular nuclei 

 of a faint bluish tint, yjuu to jo-^xto inch in diameter, 

 which sometimes project outwards beyond the plane of the 

 membrane. This structure, by the loss of the polygonal 

 outlines of its cells, readily passes into a thin membrane 

 with scattered nuclei, of just the same size and appear- 

 ance as those in the well-defined cells. If this were the 

 only change, no difficulty concerning the ectoderm of the 

 subdermal cavities need be felt ; but in some places the minute 

 C-and S-shaped spicules of the mesoderm are plainly imbedded 



