298 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
0°77 mm. to 0°9 mm. in length in the largest specimens, and is 0:035 mm. in 
diameter at the thickest point. The cladi are 0°236 mm. long in full-grown 
specimens and 0:03 mm. in diameter at the base. 
There are present also a few small plagiotrizenes, which are apparently 
young specimens of the regular orthotriznes. 
(8) Oxea. (Text-fig. 2, B.) 
(i.) The large oxea of the main skeleton are quite straight, and thickest in 
the middle of their length. They form the main part of the radial skeleton, 
and the whole of it a little way below the cortex, but they never actually 
project into the cortex itself. The largest specimens considerably exceed in 
length the rhabdomes of the orthotrizenes, sometimes reaching a length of 
1:75 mm., though they do not exceed the triawnes in diameter, being usually 
about 0°031 mm. thick. Oxea may be found in all stages of growth, from 
quite small specimens up to the largest. 
(il.) Quite distinct from the above oxea, much smaller ones can be found 
scattered irregularly, and rather sparsely, through the cortex and choanosome. 
They can be immediately distinguished from small specimens of the oxea of 
the main skeleton by the fact that they do not lie radially, but are irregularly 
scattered, quite without orientation. They are much more slender than the 
larger oxea, and measure 0°2 mm. to 0°24 mm. in length by 0:011 mm. in 
diameter in the largest specimens. 
B. Microscleres. 
(a) Sterrasters. 
The sterrasters are almost spherical and, when fully grown, form an 
almost perfectly solid mass, on the surface of which it is only just possible to 
distinguish the actines. There is a distinct but not conspicuous hilum. No 
immature sterrasters could be distinguished in the cortical sterraster-crust ; 
but in the choanosome, where the sterrasters also occur in considerable 
numbers, the proportion of developing specimens was extremely high. 
Hrom this it would seem that the sterrasters are formed in the choanosome 
and transferred to the cortex ; if so, it would indicate a continually increasing 
thickness of the sterraster-crust, or a continual wearing away of the external 
regions of the crust, in all probability the latter. 
The diameter of the fully formed sterraster is 0°06 mm. to 0°07 mm. 
(@) Spherasters. (Text-fig. 2, D.) 
The dermal spherasters are extremely small and irregular, about 5 in 
diameter, of which the centrum occupies one-half, or 2°5 wu, the individual rays 
being 1:0 to 1:5 w long, asa rule. The spherasters only occur as a dermal 
layer, and not in the deeper parts of the cortex or choanosome. 
(vy) Oxyasters. (Text-fig. 2, C.) 
There are typical oxyasters sparsely scattered through the choanosome 
