308 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
the cortex, it being very rare to find a head in the choanosome. The rhab- 
domes are similar to those of the protriznes, but even longer, and they 
frequently possess a swelling just below the head. The length of the rhab- 
dome varies from 2°5 mm. to 3°6 mm.; it is very slender, rarely exceeding 
0-004 mm. in diameter, and frequently does not measure more than 
0:0025 mm. The swelling below the head, when present, usually occurs about 
0-013 mm. down the shaft, and is about 0°006 mm. to 0:01 mm. in diameter. 
The cladi composing the head are rather short and slender ; they are straight 
in their distal portions, which make an angle of about 60° with the rhabdome. 
They are usually obtusely pointed. Their average length is about 0:02 mm. 
to 0:026 mm. and their diameter 0°003 mm. at the base ; the sagitta of the 
head is 0°02 mm. to 0°:025 mm., and the chord 0:042 mm. in full-grown 
specimens. 
Like the protriznes, the anatrizenes do not always lie regularly in the 
spicular bundle, their extreme length in proportion to their thickness render- 
ing them specially flexible. They are frequently found outside any spicule- 
fibre for a portion of their length. 
(y) Cortical trizenes. (Text-figs. 5 & 6.) 
The cortical trieenes, which are characteristic of the species, are extremely 
-rregular, and present extraordinary modifications and abortions. One or 
more of the rays may be greatly stunted, or even entirely absent, or they 
may be greatly prolonged, abruptly bent at sharp angles, covered with knobs 
and excrescences, or even branched. 
They do not occur in large numbers, and are entirely confined to the 
cortex. They are arranged quite irregularly, both as to frequency and 
orientation. 
Two main types of malformation can be distinguished. In the first (text- 
fig. 5) all four actines can be distinguished, though one or two of them are 
frequently dwarfed, being sometimes only represented by knobs. The angles 
at which the rays meet vary very much, and the rays are sometimes all 
in one plane, but it is usually possible to recognise the spicule as a true 
trizene. 
The other type (text-fig. 6) usually only possesses two of the four actines, 
though sometimes one or both of the others are present. One ray is long, 
more or less regular and straight, and usually pointed. The other ray (or 
rays) are dwarfed, and nearly always covered with knobs or protuberances, 
and form a kind of head to the spicule. The whole spicule may, in fact, be 
compared to a “freak” walking-stick. The head nearly always forms a 
right angle with the long ray. The whole spicule bears a striking resem 
blance to walking-sticks. 
It must be noted, however, that no hard-and-fast line can be drawn 
between the two types as described above, though they are separated for 
