752 
DE. J. S. BOWEEBAJs^K ON THE ANATOMY 
ferent directions. They appear to be perfectly solid ; I could not by the aid of polari- 
zation discover the slightest indication of a central cavity. They vary in diameter in 
ditferent species of sponge, and frequently even in the same individual. In a species 
of Stematumenia from the Mediterranean, I measured an average-sized fibre which was 
inch in diameter, while a smaller one, closely adjoining, measured 93^ - 5 inch. In 
this genus these fibres are more fully developed and larger in size than in any other 
sponges with which I am acquainted. In the sponges of commerce, in the membranes 
of which they are exceedingly numerous, they are much more slender. In one of the ex- 
current canals of the common honeycomb sponge, one of the largest measured ^0000 inch 
in diameter, and one of the smallest yyIt? inch. In the dermal membrane of the best 
Turkey sponge they were still less, not exceeding ^gQ o o inch. 
This description of fibre is not an absolutely necessary constituent of a sponge, and in 
many of the Halichondraceous tribes it is exceedingly difficult to find even a single 
straggling fibre on the interstitial or dermal tissues, while in other genera, as in Spongia, 
Stematumenia^ and Alcyoncellum^ they form an important element in the structure of the 
compound membranous tissues, in which they are closely disposed in parallel lines, 
occasionally giving off branches, but never appearing to anastomose with each other like 
the larger fibres of the skeleton. 
These fibro-membranous tissues were described by me in the ‘ Annals and Magazine 
of Natural History,’ vol xvi. p. 406, plate 14, figs. I, 3, 4 & 5, in my description of the 
genus Stematumenia. 
If a small portion of the dermal membrane of a young Stematumenia be carefully 
removed from the surface of the sponge, the primitive fibres will be seen projecting from 
the edges of the membrane in considerable numbers ; and occasionally they may be seen 
to be furnished with a terminal bulb, the greatest diameter of which is about three 
times that of the fibre. The bulbs are variable in form ; sometimes they are largest at 
the base, or pear-shaped, at other times regularly oval, or nearly globular. By far the 
greater number of fibres exhibit no bulbs at their terminations ; those which have them 
are always less in diameter than the general average of the fibres. Sometimes, but not 
very frequently, the bulb exhibits faint traces of a nucleus. On examining the dermal 
membrane by transmitted light and a linear power of 666 , I found numerous globular 
cells collected in groups on various parts of its inner surface, many of them having a 
well-defined central nucleus ; and among these cells I found the bulbs imbedded with 
the fibres emanating from them, and in no respect differing in appearance from the non- 
fibrous cells around them (Plate XXVII. fig. 5, «, a). On carefully observing a number 
of these bulbous fibres that had been removed from their positions on the membrane, I 
found that the part of the fibre nearest to the bulb was frequently flexuous, as if in a 
tender and immature condition, and in these cases the marginal line of the fibre was 
continued without the slightest break or interruption into and around the bulb, as 
represented in Plate XXVII. fig. 6 , a. At this period of the development the yoimg 
fibre does not measure above half the diameter of a mature one, and there is no indica- 
tion of an ultimate separation from the bulb ; but when the fibre has attained nearly the 
