SILICIOtJS AND HORNY SPONGES WILSON. 493 



number of canals debouch. The oscular depressions may, however, 

 be absent, the several canals opening; independently on the surface. 

 The body is greatly excavated by canals, the larger of which are 

 4-5 mm. in diameter. The dermal membrane is filled with broken 

 spicules and a few sand grains, constituting the usual " sand cortex." 

 This is distinct but thin, in general 100 p. or less in thickness. In 

 certain regions the mineral bodies are uniformly distributed through 

 the membrane, which in such places shows no signs of the pores. 

 But over the greater part of the surface they are aggregated to 

 form a dense reticulum in the meshes of which there is only thin 

 membrane quite lacking such bodies. Here and there perforating 

 the membrane filling a mesh is found an open pore. The meshes 

 of this cortical reticulum are about 140 [x in diameter, the intervening 

 strands about 50 fx thick. The entire arrangement indicates that 

 when the pores have been closed for some time the mineral bodies 

 become uniformly distributed through the dermal membrane, but as 

 the pores open these bodies are shifted and become concentrated be- 

 tween and around them. The cortical reticulum is visible to the eye. 



The filaments are very abundant, about 5 ;x thick, tapering to one- 

 half that thickness at the ends, where the enlargements are spheroidal 

 and 10-12 pi in diameter. They are unspotted. The sponge in the 

 dried state is compressible. Color purple or purplish gray on upper 

 surface, yellowish brown on attached surface. 



Skeleton. — Main fibers and connectives are distinguishable in the 

 peripheral part of the spong-e, the main fibers terminating in the 

 conuli. But in the interior the course of the main fibers becomes so 

 irregular that the skeleton here is not distinctly divisible into such 

 fibers and connectives. Doubtless the larger fibers of the interior, 

 with fairly abundant foreign contents, represent the main fibers, 

 but no order is distinguishable in their distribution. 



The main fibers {in. f. in figs. 2, 4, 6, 7 of pi. 52) may be only 

 slightly or distinctly " fascicular,"' in the characteristic Hircinia- 

 fashion as contrasted with Stelosporu/la — that is, the " fascicular " 

 state is here produced by the prolongation along the primitively 

 simple main fiber of the roots of connectives, instead of by the bind- 

 ing together of several more or less parallel fibers into a compound 

 fiber. (See George and Wilson, 1919, pp. 168, 171.) The originally 

 simple, solid, main fiber is about 150 pi. thick, containing abundant 

 sand grains and spicule fragments. It may remain simple through- 

 out the greater part of its extent, being fascicular (fenestrated would 

 be a better word) only where the roots of connectives meet it (pi. 

 52, fig. 4) ; or the connective roots may spread along the fiber to such 

 an extent that it becomes almost everywhere fenestrated (pi. 52, 

 fig. 6) ; such fibers mav attain a thickness of 300 (x, occasionallv even 

 400 pi. 



