314 MARINE BIOLOGY OF THE SUDANESE RED SEA. 
the cladi of the regular head are frequently malformed, with knobs and small 
excrescences growing on them. In exceptional cases the cladus may be bent 
at a sharp angle part way along its length. Hxamples of these malformations 
are shown in text-fig. 8, the two spicules at the top of the figure being normal 
perfect examples, those in the second row malformed spicules projecting from 
the sponge surface, and those in the lowest row similar spicules whose heads 
were buried in the sponge-body. The rhabdomes of both anatriznes and pro- 
trieenes are very slender and flexible, and frequently do not lie parallel with 
the oxea, but sinuously in the sponge. 
The protrienes are somewhat few in number, especially with their heads 
inside the sponge-body, and almost all of them are very much malformed. 
In fact, only a single specimen was seen in which all three rays were present 
and well developed. The length of the rhabdome varies very much, and 
specimens may be found with the rhabdome anything up to 1:5 mm. in length. 
It is, of course, quite possible that the broken spicules measured much more 
than this when complete. The thickness of the rhabdome is 0:005 mm. in the 
largest specimens at the head, and the rhabdome tapers from this point gradu- 
ally to an extremely fine point. The cladi naturally vary very much in 
length, the longest cladus found being 0:084 mm. The chord of the head 
also varies with the length of the cladi and the amount of malformation the 
head has undergone, and no average measurements can be given. 
The anatrienes, which are fairly plentiful, possess a rhabdome very similar 
to that of the protriznes. It varies considerably in length, some specimens 
being found to attain a length of 2:1mm. The diameter is greatest close to 
the head, where it measures 0-004 mm., and from this point the rhabdome 
gradually tapers to the point. It is almost impossible to give measurements 
for the head, owing to the malformations which occur, but the following are 
taken from a complete and normal specimen :— 
Cladi 0°043 mm. in length, 0:006 mm. in diameter at the base. 
Sagitta 0:035 mm. 
Chord 0:06 mm. 
One of the most frequent malformations, that in which the head is reduced 
to a knob, seems to be confined entirely to anatrienes. This knob usually 
measures from 0°013 to 0-018 in diameter. 
(8) Oxea. 
(i.) The oxea of the main skeleton are long, pointed at both ends, and much 
stouter than the trienes. They are thickest in the middle of their length, and 
taper gradually and uniformly to each end. Those oxea which have one end 
projecting from the surface of the sponge are practically all broken, so that 
their length cannot be ascertained, but there does not seem any reason for 
