AND PHYSIOLOGY OF THE SPONGIAD^. 
753 
full size the separation is then distinctly indicated ; the basal end of the fibre immersed 
in the hulb becomes hemispherical, and a constriction appears at the junction of the 
fibre mth the exhausted cell. Sometimes, when thus afibrding indications of their ulti- 
mate separation, the cell still retains its rotundity, but all indication of its nucleus has 
disappeared, and it is perfectly transparent, as represented in Plate XXVII. fig. 6, h ; 
while in other cases it is visible only as a collapsed and shrivelled vesicle adherent to 
the hemispherical termination of the fibre, as represented in Plate XXVII. fig. 6, c. I 
could not find the slightest indication of bulbs amid the matted mass of fibres lying on 
the inner surface of the membrane, and it was only at the torn edges of the pieces of 
membrane under examination, or among the groups of cells, that the bulbs in connexion 
with the fibres were to be discovered. 
2. Keratose Fibrous Tissue. 
General character of the keratose fibres of the horny skeleton. — The essential character 
of the fibres of the horny skeleton is, that their normal form is always that of a cylinder, 
while the network of the skeletons of the Halichondroid sponges, which approach 
nearest in structure to that of spiculated keratose fibre, is always more or less irre- 
gular in shape ; and in the fully developed state, generally compressed to a very consi- 
derable extent ; but a careful examination of the youngest portions of the two forms of 
skeleton-tissue will always render the difference in the two structures apparent. In the 
spiculated keratose fibre the keratode is always the predominant element, and the spicula 
the subordinate one ; while in the skeletons of the Halichondroid sponges the spicula 
always predominate, and the keratode is merely the secondary or surrounding medium. 
In the former structure, in the extension of the terminations of the skeleton, the keratode 
is the leading element, while in the latter the spicula take the lead. 
The fibre is formed of a succession of concentric layers, its increase in diameter being 
apparently effected at the external surface. Its longitudinal extension appears to be 
caused by a progressive elongation of their terminations, and new fibres are frequently 
to be seen pullulating from the sides of the mature ones. In the dried state it is often 
extremely rigid and incompressible, but m its natural condition, notwithstanding there is 
frequently an internal axis of extraneous matter or of spicula, it is often remarkably soft 
and fiexible. The spicula, although immersed in the fibre, evidently possess a consider- 
able amount of mobility within the surrounding medium. 
The colour of the fibres is always amber-yellow, varying in different species from a very 
light to a deep yellow brown tint, and it is always semitransparent. In the living state, 
when the fibres happen to touch each other, whether by their terminations or laterally, 
they appear at all times to unite. 
The keratose skeleton-fibres vary in their organization to a very considerable extent, 
but the whole of them may be comprised in the following eight typical forms : — 
1. Solid simple keratose fibre. 
2. Spiculated keratose fibre. 
