350 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
Skeleton arrangement. (PI. 40. fig. 26.) 
In general, the skeleton arrangement is very similar to. that of Ophlita 
spongia arbuscula, above described. ‘There is the same dark-orange spongin- 
fibre feebly cored with monactinal megascleres, and the same large numbers 
of spicules scattered about throughout the sponge. There is also present 
a dermal skeleton, even more dense than in the former species. It consists 
of brushes of spicules, whose ends project from the surface, and which are 
densely matted together. There are, in addition, large numbers of spicules 
lying in a tangential position, and others which occupy positions intermediate 
between the radial and tangential, so that the whole makes a very dense 
felt-like covering to the exterior of the sponge. Frequently, also, large 
sand-grains occur either enmeshed in the dermal skeleton or else lying 
immediately below it. 
b 
Fig. 23.—Ophlitaspongia horrida, Spicules, x 315. 
The reticulation of the spicule-cored spongin-fibre of the main skeleton 
is much less regular than in O. arbuscula, and the mesh is elongated and 
varies so much in size that it is almost impossible to give an average. As 
limits 0-2 mm. and 1:5 mm. may be suggested. 
The fibres cannot be differentiated into primary and secondary, either by 
their spiculation or their size. They vary very little in size or in the 
number of spicules they contain, the diameter of the fibre being usually 
0:09 mm., and the spicules in the fibre being 4- to 7-serial. The spicules 
are arranged in a very slightly plumose manner. 
