756 
DE. J. S. BOWEEBANK ON THE ANATOMY 
occasionally it is dilated considerably for a short space, and then resumes its original 
diameter. In the young state the cavity is as large, or nearly so, as in the adult fibres, 
while the enveloping keratode assumes the form of a thm transparent amber-coloui’ed 
coat, which in the mature state becomes frequently twice or three times the thickness of 
the diameter of the central cavity. 
This great fistular space is lined with a thin pellucid membrane, which, in specimens 
that have been diied, appears to have been thickly covered with minute semi-opake 
granules. At the time of my first description of this form of fibre, published in the 
‘ Annals and Magazine of Natural History,’ vol. xvi. p. 403, I believed that in the natural 
condition of the fibres the central cavity was an open tube ; but subsequent observations 
on specimens which have never been dried, have led me to the conclusion that the whole 
of the central space is filled with a minutely granulated substance which presents all the 
characteristics of sarcode. 
There is no communication between the great central fistular canal and the inter- 
stitial cavities of the sponge, the projecting ends of the fibres of the skeleton being 
always hermetically sealed. Fig. 12, Plate XXVII. represents a fibre from the spe- 
cimen of Spongia fistularis, Lamaeck, in the Museum at Edinburgh, given to me by 
Professor Geant. 
6. Compound Fistulose Keratose Fibre. 
In its external characters this description of fibre is not, under ordinary circumstances, 
to be distinguished from simple fistulose fibre, and it is only when submitted to a micro- 
scopic power of about 100 linear that its peculiar character can be detected. We then 
find that the fibre is not only furnished with a large continuous central cavity, but that 
it also has numerous minute ceecoid canals radiating from the central one at irregular 
distances, at nearly right angles to its axis. These secondary canals are very unequal in 
length, and very few of them reach to near the external surface of the fibre, and none of 
them appear to perforate it. Their direction is usually in a straight line from the parent 
canal ; a few assume a tortuous direction, and a still fewer number bifurcate or branch. 
Within the central tubes of the fibres there are frequently one or two minute simple 
tubular fibres ; when more than one they do not unite, but they divide and traverse each 
a separate cavity, when they happen to reach one of the anastomosing points of the great 
skeleton-fibre. The structures are described more at length in the ‘ Annals and Magazine 
of Natural History,’ vol. xvi, p. 405, under the head of ’■’■AulisJciaF a new genus of sponges, 
founded principally on the compound fistulose structure of its skeleton-fibres. Fig. 13, 
Plate XXVH. represents a portion of compound fistulose keratose fibre as seen with 
a linear power of 100, Fig. 14, a portion of a similar fibre under a power of 300 
linear. 
7. Regular Arenated Keratose Fibre. 
This description of fibre under ordinary circumstances has very much the appearance 
of simple fistulose fibre, but when examined by transmitted light with a linear power of 
