R. W. H. ROW—REPORT ON THE SPONGES : NON-CALCAREA. 297 
probability those oscula which do not show this membrane possess it, but in 
those cases it is rendered invisible by being widely open. 
There also occur, scattered over the sponge surface, and apparently no. 
-more frequent on the under side of the sponge than on the upper, a number 
of much smaller apertures, over the great majority of which there stretches 
a membrane. ‘These are in all probability pore-areas ; but microscopical 
examination of the covering membrane has not revealed any pores, so that 
these apertures may possibly be closed oseula. The diameter of these small 
apertures is about 0-1 mm. 
The colour in spirit is gleaming white on the outside, and white with a 
tinge of yellowish grey in the interior. 
The ectosome is clearly differentiated into a cortex, which is about 1-0 mm. 
to 1-2 mm. thick and which is distinctly fibrous in the region below the 
sterraster crust. Owing to the density with which the sterrasters are packed 
in the outer portions of the cortex, the character of the cortex cannot be made 
out in that region. The cellular elements, if there are any occurring in the 
cortex, were not distinguishable, and the cortex was quite hyaline. 
Skeleton arrangement. (PI. 40. fig. 24.) 
The cortical skeleton consists of a dermal layer of minute chiasters, very 
thickly scattered over the whole sponge surface, and especially thickly lining 
the sides of the vents. Below them lie the sterrasters, which are packed into 
a solid mass 0°6 mm. to 0°8 mm. thick, extending over the whole sponge 
surface, and only interrupted at the oscula (and at the presumed pore-areas). 
The main skeleton is radial, but only partially composed of spicular fibres, 
large numbers of spicules lying separately in the choanosome. The spicular 
bundles which do occur are, however, frequently very large, sometimes as 
much as 0-5 mm. in diameter. They are very deeply packed together, and 
in the deeper parts of the sponge consist almost entirely of oxea, the heads of 
the trizenes lying either in the deeper parts of the cortex or immediately 
below it. 
Spicules. (Text-fig. 2.) 
A. Megascleres. 
(«) Triznes, (Text-fig. 2, A.) 
There only occurs one form of trizene in this species, an almost typical 
orthotriene. The rhabdome is straight, thickest at the junction with the 
eladi, and gradually tapering to the point. The cladi are somewhat long, as 
stout as the rhabdome, and very slightiy recurved. Like the rhabdome, they 
also taper from base to point. The heads of the trieenes never lie within the 
layer of sterrasters, but always either in the deeper parts of the cortex 
or in the more superficial parts of the choanosome. The rhabdome measures 
