344 MARINE BIOLOGY OF THE SUDANESE RED SHA. 
areas. In this case, however, the sieve-membrane stretched over the pore- 
area is replaced by a sphincter-membrane. The oscula usually measure 
about 1°5 mm. to 2:0 mm. in diameter. 
The colour is yellowish white in spirit. 
The texture is firm, resistant, and hardly at all compressible. 
Skeleton arrangement. 
The main skeleton consists of a very dense and quite irregular mass of 
spined styli, lying in the sponge entirely without orientation. Here and 
there a few spicular fibres can be seen, but these fibres are very few and are 
not oriented, so that it seems probable they are but 
accidental, owing to the spicules lying parallel in- 
stead of irregularly at that particular spot. 
At the surface of the sponge tylote spicules are 
met with. In the actual dermal surface, between 
the “ pore-cones,’’ both tylota and acanthostyli occur, 
promiscuously mixed with each other, and tylota are 
also found below the surface, but alwavs near it. 
The pore-cones are supported by a special skeleton, 
composed entirely of tylostyles, and the oscular cones 
have an exactly similar skeleton. The wall of the 
cone is supported by numerous dense bundles of 
tylota, arranged vertically in the cone-wall. These 
bundles are about 0°15 mm. in diameter, and the 
spicules in them are very densely packed. They are 
separated from each other by a width approximately 
equal to the diameter of the bundle. In the sieve- 
membrane there frequently occur single tylota and 
large numbers of chele, but there is no special 
Fig, 20.—Mywilla cratera skeleton. 
Spicules, x 650. 
Spicules. (Text-fig. 20.) 
A. Megascleres. 
(i.) Tylota. (Text-tig. 20, D.) 
Rather long, slender tylota, with a slight but curiously shaped head. In 
no part is the head of much greater diameter than the shaft of the spicule, 
but it is quite definitely, if slightly, larger than it; but instead of being 
rounded off on the distal side, the head is occasionally almost sharply 
pointed. The tylota are confined to the dermal membrane and adjacent 
parts. Length 0°23 mm. on the average; diameter 0:003 mm. Diameter 
of head 0:004 mm. 
