302 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
lie tangentially, but otherwise without orientation, and the layer is very 
dense. This spicular layer is about 0-2 mm. thick, and lies 0°5 mm. from the 
surface of the sponge. 
Dermally also there is a dense surface-layer of small spherasters, among 
which considerable numbers of the large sterraster-like spherasters occur. 
Below the cortex the skeleton is very scanty, consisting solely of somewhat 
sparsely scattered oxea, which le in the sponge-body entirely without 
orientation and without relation to each other. Both small and large 
spherasters also occur throughout the sponge. 
Spicules. 
A. Megascleres. 
The only megascleres occurring in the sponge are oxea, and only one kind 
of these is present. They form both the tangential cortical skeleton and the 
scattered spicules of the choanosome. They are usually quite straight and 
are thickest in the centre, tapering very gradually towards the ends for most 
of their length, and more rapidly at each end to a sharp point. The spicules 
vary very much in size, and spicules in all stages of growth can be found ; 
the average full-grown specimen measures from 0°85 mm. to 1:0 mm. in 
length, with a maximum thickness of 0:016 mm. 
Certain malformations and irregularities occur among these spicules. Itis 
occasionally found that a spicule, instead of being quite straight, is distinctly 
bent, with a distinct angle at the centre. Again, there may be found, more 
rarely, specimens with one end abruptly rounded off, thus making the spicule 
apparently a style. These are, however, quite rare, and the presence of other 
malformations points to the conclusion that the rounded ends are accidental. 
The other malformations occur in the shape of extra actines, usually added 
near the end of the spicule, but sometimes one-fourth of the length of the 
spicule from its end. These supernumerary actines are usually very short, 
sometimes being merely a knob on the shaft of the spicule. The longest 
I have seen measured 0°05 mm. in length. ‘They are all inclined to the ray 
at an angle of 35°-40°. 
B. Microscleres. (Text-fig. 4.) 
(a) Spherasters. 
(i.) A large spheraster which, when fully grown, simulates a sterraster, 
occurs throughout the sponge-tissues, but most frequently in the superficial! 
regions of the cortex. The smallest seen examples are typical oxyasters with 
4 or 5 rays, and measuring 6°5 w in diameter. Between these minute 
spicules and the fully-formed spheraster a complete series of intermediate 
forms occur, clearly showing the mode of growth of the spicule. The rays 
of the young aster grow in length and rapidly become very numerous, 
