of the Fibre in the Spongida. 119 
Thus, then, my study of the development of the fibre leads 
me to the inference that the granular core is able to produce a 
kerasine layer at first, but that subsequent ones are added by 
some other agent of the sarcode or general parenchyma ; while 
the kerasine is supplied by the pigment-cells in Janthella 
simulated by faint cell-structure in Aplysina purpurea, but 
in no other instance that has come under my observation have 
I been able to see this. 
Other facts bearing upon the fibre and the spicule respec- 
tively might be mentioned here with advantage, viz. that the 
interior of a Rhaphidonematous fibre may have the whole of 
its spicules removed by absorption, and the core so transformed 
into a simple granuliferous tube, while the horny part still 
remains unaftected, that it becomes almost identical with the 
fibre of Aplysina, and that, too, while the acerate spicules in 
the circumferential fibre remain intact, as I have before men- 
tioned (p. 109)—which led to my calling the specimen “Aply- 
sina chalinotdes.” 
Again, it is not uncommon to find the core-spicules in both 
the Rhaphidonemata and Echinonemata only partly absorbed, 
although the horny fibre in this case also remains perfectly 
intact. Here the spicule is often obliterated, all but the cen- 
tral canal and a single fragment of its entire calibre in the 
centre, whereby it presents the form of a spindle—which at 
first appears to be a new form, butis subsequently proved, by 
the presence of others in different degrees of absorption, to be 
otherwise, and the true form of the spicule thus found out. 
Nor is it uncommon to find the central cores of spicules 
themselves so enlarged that the siliceous portion is more or 
less reduced to a mere continuous film while its extremities 
are still closed. 
All this points out that the spicules within the fibre and the 
internal part of the spicule itself may undergo absorption with- 
out any evident contact with the element by which they may 
be surrounded. 
I have said nothing of the giassy fibre of the vitreous Hex- 
‘actinellida, because it, mutatis mutandis, is the same as the 
horny fibre; and, of course, in the Lithistina there is no fibre 
at all, where its office is supplied by the interlocking of the 
filigreed extremities of the branches of the spicules. 
EXPLANATION OF PLATE IX. 
Fig. 1. Aplysina purpurea, from Trincomalee. Upper half of the speci- 
men, natural size. aa, vents; 6, monticular elevation, magnified 
2 diameters; c, reticular subdermal structure; d, dermal termi- 
nation of fibre; e, group of pigmental and horn-cells; f, pig- 
mental cells; g, horn-cells (scale 1-48th to 1-6000th inch) ; 
