524 Revision of the AXiNEiiLiD^, ii., 



small oval or circulai' groups, averaging about IdO/x in diameter 

 but somewhat variable in size, scattered over the entire surface, 

 and containing usually less than 10 pores each. Where the 

 dermal membrane has disappeared, the surface is closely per- 

 forated with minute pinhole-like apertures, which are the open- 

 ings of the inhalant canals : the presence of these is discernible 

 also where the dermal membrane is intact, but, as a rule, only 

 faintly and indistinctly. 



Skeleton.— The skeleton which remains, after complete macera- 

 tion of a specimen by means of caustic potash, preserves exactly 

 the external form of the perfect sponge; it is composed of pale- 

 coloured, highly sponginous fibres, and is fine-textured and of 

 sufficient density to render it difficult for one to perceive from 

 external inspection whether a condensed axial region is present 

 or not. In section, under the microscope, the pattern is seen to 

 be fairly regularly reticulate, the reticulation being formed by 

 longitudinal and obliquely outward-trending main fibres pauci- 

 serially cored with spicules, and by numerous short connecting 

 fibres containing no spicules (PI. xxxiv., fig.l). The reticulation 

 is condensed axially, though not in any very marked degi-ee 

 except in the older, more basal parts of the branches, the con- 

 densation being the result merely of a progressive increase of 

 stoutness of the fibres, — most rapid in connection with the 

 axially situated ones, and scarcely at all aft'ecting those situated 

 near the periphery, — with increasing age. Within the axial 

 region of the oldest part of the branches, the fibres may attain 

 a stoutness of over 100//; but throughout the greater part of the 

 skeleton, they are comparatively slender, even the main fibres 

 seldom exceeding 40/ji, while the connecting fibres are of all 

 degrees of lesser stoutness down to below 5/x. Irregularity in 

 the pattern of the skeleton is due to the fact that the connecting 

 fibres rarely pass singly and directly between the main fibres (in 

 such manner as to produce a rectangular or scalarifoim reticula- 

 tion), but to a greater or less extent, — depending on the distance 

 apart of the main fibres, — interunite among tliemselves, thus 

 giving rise to an irregularly- meshed, somewhat plexiform reticu- 

 lation. The avei-age width of the meshes is less than 100/a, 



