312 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
about 0°75 mm. in diameter and scattered over the sponge surface quite 
irregularly. The pores were not distinguishable. 
The colour in spirit is yellowish to dull yellow-brown. 
The ectosome is quite clearly marked off from the choanosome; it is 
perfectly hyaline in appearance and no cell-elements could be found in it. 
The average thickness of the ectosome is 0°5 mm. There is no special 
cortical skeleton whatsoever, unless the layer of sand-grains referred to above 
is considered to fall under this head. 
The skeleton is radial, and most of the spicules are arranged in rather 
loose bundles, though many spicules occur lying free in the choanosome 
between the spicule bundles. 
In the deeper parts of the spozge the spicule bundles are fairly compact, 
but the spicules are quite separate in the outer regions of the sponge. When 
the actual surface of the sponge is reached, the spicule bundle is expanded 
into a brush-like head, and the spicules of adjacent bundles become inter- 
mingled. Thus the whole surface of the sponge is covered by an even coat of 
spicule heads. These spicular bundles measure from 0:12 mm. to 0-14 mm. in 
diameter when the spicules composing them are close together, as they are 
near the centre of the sponge. The whole of the spicules in the main skeleton 
in the deeper parts of the sponge are oxea, the anatrizenes and protrizenes being 
confined to the superficial parts; in fact, it is quite rare to find a trizene head 
more than 2 mm. from the sponge surface. 
Scattered about through the choanosome occur large numbers of small 
oxea, closely resembling those of Tetilla poculifera, Dendy, referred to above. 
There are now, in fact, six species of Tetillidee in which these microxea 
occur now known to science, namely: Zetilla australiensis, in which they 
are minutely spined, and Tetilla poculifera, Chrotella ibis, and all the three 
species of Paratetilla at present described, P. merguiensis (Carter), P. cineri- 
Jormis, and P. eccentrica, in all of which they are smooth. It is also note- 
worthy that in two species— Tetilla poculifera and the species at present being 
described, Chrotella ibts—they occur in enormous numbers, so that the colour 
of the specimen is noticeably affected. In the present species they do 
not occur so thickly as in Yetilla poculifera, but nevertheless they form a 
conspicuous part of the whole skeleton. They are entirely confined to the 
choanosome, the cortex being absolutely free from them. 
Spicules. 
A. Megascleres, 
(2) Trieenes. (Text-fig. 8.) 
Trizenes are present in considerable numbers in the sponge. They forma 
very considerable proportion of the spicules projecting from the sponge 
surface, and although these projecting spicules are very frequently broken 
