342 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
Spicules. 
A. Megascleres. 
The only megascleres occurring in this sponge are subtylostyli, and there is 
no difference either in shape or measurement between the spicules of the 
dermal reticulation of spicule fibre and those of the main skeleton. 
They consist of a long slender shaft, with a very slightly developed head 
at one end, and have a very fine point at the other. The thickest part of the 
shaft is about halfway along the spicule, and from this point the shaft 
gradually tapers towards either end. ‘The head is very small and oval in 
shape, with its longer axis a continuation of the axis of the spicule-shaft. 
The length of the spicule varies from 0°32 mm. to 0°33 mm., and the 
average diameter of the shaft at its thickest point is 0004 mm. The diameter 
of the head varies from 0-004 mm. to 0°0045 mm. 
B. Microscleres. (Text-fig. 19.) 
(i.) Chele. (Text-fig. 19, A.) 
The chelz are confined almost entirely to the dermal membrane, where 
they occur in conspicuous rosettes, measuring 0°07 mm. in diameter. The 
chelee are palmate anisochele, and their length is usually from 0:024 mm. to 
0:036 mm. The width of the chela is on the average 0:0115 mm. 
A large number of small chelze were seen, some being isochele, and 
varying very much in size, in all parts of the sponge. They do not form 
rosettes, and were usually scattered about singly. Their average length is 
0-015 mm. 
(ii.) Sigmata. (Text-fig. 19, B.) 
The sigmata are not very plentiful, but occur in all parts of the sponge. 
They are usually C-shaped, but also can be found contort. They vary con- 
siderably in size and to some extent in shape. They measure from 0°05 mm. 
to 0:075 mm. across the curve of the sigma, and their diameter does not 
exceed 0-002 mm. 
Gis) eloxas i (exits ©.) 
The toxa are very few in number, and only occur in the deeper parts of 
the sponge. They are strongly bowed, and very slender throughout their 
length, so much so that at the extremities it is very difficult to determine the 
ends of the spicule with accuracy. Their average length is 0°09 mm., and 
their diameter less than 0°0005 mm. 
(iv.) Raphides. 
Numerous very slender hair-like oxea occur throughout the sponge. It is 
not possible to say for certain whether they are young oxeote forms of the 
regular styli or true raphides. They vary in length from 0:02 mm. to 
0:15 mm. Their diameter rarely exceeds 0°:001 mm. : 
Locality. Khor Shinab. 
Distribution. Red Sea. 
