480 E. A. MTNOHIN. 



may have been tlie means of spreading, I think it necessary to 

 reaffirm the views which I have held ever since I began to 

 study the histology of the Ascons, and which I still hold more 

 strongly than ever ; namely, that in the normal expanded 

 condition of the sponge the dermal layer is limited at its free 

 surfaces by a flat epithelium, of the usual type of epithelia to 

 which this term is applied ; that this flat epithelium, in the 

 family Clathriuidse at least, is the contractile or neuro- 

 muscular layer of the sponge; and that when completely 

 contracted the epithelial cells alter in form, becoming mush- 

 room-shaped everywhere, — that is to say, except in the interior 

 of the oscular rim. 



The cells of the dermal flat epithelium have a very charac- 

 teristic appearance and structure in all the species. Seen in 

 surface view (PL 38, figs. 1, 3, and 10; PI. 39, figs. 16 and 20; 

 PL 41, fig. 40, &c.), the nuclei are seen scattered about, each 

 in the centre of a patch of very granular protoplasm, in 

 which the nucleus is sometimes almost hidden. Cell outlines 

 caa often be made out as very delicate bright lines dividing 

 the surface into polygonal areas, but it is very difficult 

 to discern them as a rule, on account of the refraction of 

 the underlying spicules. The nucleus of the epithelial cell 

 is spherical and relatively large, usually causing the surface 

 to bulge out, as seeu in sections (PL 41, fig. 41). It con- 

 tains a fine and more or less even reticular framework, with 

 chromatiu at the nodes, and usually a small nucleolus placed 

 somewhat excentrically. Stained with picrocarmine after 

 fixation with osmic acid, it appears evenly stained or finely 

 granular, with a darker spot representing the nucleolus. 

 Its absolute size varies in diff'erent species, as is the case with 

 the nuclei of all the other tissue elements; Clathrina con- 

 torta is especially remarkable for the large size of its cells, 

 and the nuclei of its flat epithelium are nearly twice as large 

 as those of coriacea, as may be seen by comparing PL 41, 

 fig. 40, and PL 39, fig. 16. The relative size of the nuclei of 

 the various tissue elements in each species is a much more 

 constant character, and especially the proportion between the 



