348 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
Skeleton arrangement. (Pl. 40. fig. 25.) 
The main skeleton consists of a dense reticulation of spicule-cored fibre, 
and of spicules scattered irregularly about throughout the sponge. The 
fibres are strongly coated with spongin and the reticulation is very close. 
There cannot be made a distinction into primary or secondary fibre, no 
separation of the fibres into groups being possible either in direction or size, 
for the reticulation, although the fibres are unoriented, is fairly regular, and 
they are all of equal size. The fibres average 0:08 mm. in’ diameter, but 
where a junction between two or more fibres occurs there is a slight swelling 
into a knob, which usually measures 0°12 mm. in diameter. The spongin 
coating the fibres is made up of a series of layers, and the divisions between 
the various layers can very easily be seen. 
The dermal skeleton consists of dense brushes of spicules, typically 
arranged fanwise at right angles to the surface of the sponge, but also 
frequently almost tangential in direction. As a result, the dermal skeleton 
forms a dense felting over the surface of the sponge. 
Spicules. (Text-fig. 22.) 
A. Megascleres. (Text-fig. 22, A.) 
The whole of the megascleres are subtylostyles, and the spicules of the 
fibres are not distinguishable either in size or shape from those of the dermal 
skeleton or those scattered throughout the sponge. In 
almost all the fibres they form a slender core, being 
usually arranged 2- to 3-serially; but in some fibres 
they are uniserial, or in rare cases even entirely 
absent. They are frequently arranged in a slightly 
plumose manner within the spongin-fibre, but they 
never project outside it. Thus, although true echi- 
nating spicules are entirely absent in this species, they 
seem to retain some slight indication of Hctyonine 
affinity. 
The spicules are slender, frequently curved, and, not 
always uniformly throughout their course, they seem 
to follow the curve of the fibre. They almost always 
possess slight heads, which are oval, with the long 
axis of the head a continuation of the axis of the 
{ spicule. The size varies considerably, and spicules 
Fig. 22. in the fibres seem to attain slightly larger dimensions 
Ophlitaspongia than those outside them, but this difference is very 
arbuscula. slight and may not be so in all parts of the sponge. 
Spicules, x 250. The length of the average spicule is 0°3 mm., but 
Specimens measuring as much as 0°33 have been seen. Their diameter 
