82 BowERBANK^ ou Gvantia ciliata. 
always more or less elongated and oval, and tlie mouth of 
the great central cloacal cavity is furnished with a circular 
ciliary fringe, composed of very long, slender acerate spicula. 
We find, also, that although the cloaca gradually enlarges 
towards its distal extremity, the distal end of the parietes of 
the sponge decreases in its diameter, and this is effected by 
successive diminutions in the lengths of the interstitial cells, 
as they approach the termination of the sponge, where the 
neck of the cloaca consists of a stout membranous tissue and 
interlacing spicula only ; and here we must pause to consider 
the admirable construction of this tissue for the purposes it 
is destined to perform. The mode of the disposition of the 
equiangular triradiate skeleton spicula, in all parts of the 
sponge, allows of a very considerable latitude for expansion 
and contraction of the membranous tissues to which they 
are attached ; but in the neck of the cloaca, where a great 
amount of dilatation is necessary, there is an especial provi- 
sion for this effect, both in the form and mode of arrange- 
ment of the spicula, which are now found to be rectangular 
triradiate, and to be disposed in regular bands, having the 
two rays in the same plane of each spiculum always placed 
at right angles to the long axis of the sponge, while the third, 
or angulating ray, is parallel to the long axis, and this form, 
combined with the position, admirably adapts them for their 
purpose, for while the lateral radii allow of almost any 
amount of motion of the membranous tissues over their sur- 
faces, they are rigidly maintained, each in its own place, by 
the resistance of the angulating radii, which invariably 
remain in accordance with the long axis of the sponge. But 
this is not the sole ofiice that is effected by these curious spi- 
cula, for many of them produce a fourth or spicular ray at 
right angles from the junction of the other three, and this 
spicular ray is projected inward, and always more or less 
curved towards the mouth of the cloaca, thus becoming a 
formidable weapon of defence against the encroachments of 
intruders. The bases of the numerous and very slender 
ciliary spicula are attached to the membranous neck of the 
cloaca for more than half its length, and the exterior surface 
of the neck is strengthened and supported by numerous 
short, but very strong, fusiformi- acerate spicula. In a state 
of repose, or of gentle inhalent action only, the neck of the 
cloaca maintains its greatest amount of contraction, and the 
apices of the long ciliary spicula are all concentrated at a 
point very considerably above the membranous neck of the 
cloaca, forming a dense cone of parallel spicula, through 
which nothing but fluid could enter. But as soon as the 
