AND PHYSIOLOG-T OF THE SPONGIAD^. 
755 
the predominant element, and the spicula the subordinate one, and we accordingly 
frequently find the fibres destitute of spicula for short distances ; but these occurrences 
are the exceptions, and not the rule of the structure. Fig. 8, Plate XXVII. repre- 
sents a portion of a longitudinal section of the skeleton of Halichondria oculata, 
JOHNSTOK 
The mode of the progressive development of this form of fibre is interesting. In a 
young specimen of Halichondria Montagui, Johnston, I observed that when a new fibre 
was projected from the skeleton it usually contained a single spiculum, thinly covered 
by keratode at the apex, and more thickly so towards the basal end. Another spiculum 
followed the first, the terminations of each overlapping the other; and at the junction of 
the two, the keratode was accumulated in the form of a plumber’s joint, as represented 
in Plate XXVII. fig. 9, so as to give additional strength to the junction of the spicula, 
while the middle portion of the second spiculum remained very thinly covered by 
keratode. When the distal end of the new fibre has attained its proper length, or has 
become cemented to the side of another fibre, the remaining portion of the keratode is 
produced, and the fibre then assumes a regular cylindrical form. 
3. Multi-spicidated Keratose Fibre. 
This description of fibre is literally a cylindrical mass of spicula cemented together by 
keratode, and surrounded by a thin case of the same substance. The spicula are exceed- 
ingly numerous, and very closely packed in parallel lines in accordance with the axis of 
the fibre. They are nearly uniform in size, and always of the same shape in the same 
species of sponge. In this structure the spicula are the predominant element, and the 
keratode the subordinate one. Fig. 10, Plate XXVII. represents a fibre from the 
skeleton of Halichondria cegagropila, Johnston. 
4. Inequi-sjpiculated Keratose Fibre. 
This form of fibre is composed of an infinite number of spicula disposed in every 
possible du’ection, cemented together by keratode, and surrounded by a sheath of the 
same material. The spicula agree in form in all parts of the sponge, and are nearly of 
the same size. In these fibres the spicula are the predominant element, the keratode the 
secondary one. In the only sponge in which this form of structure has yet been found, 
Faphyrus Griffithsii, Boweebank, MS., the fibre is very unequal in size and much varied 
in its form, frequently becoming very much flattened and expanded. Fig. 11, Plate A. 
represents a longitudinal section of a small portion of a fibre from the skeleton, showing 
the irregular disposition of the spicula within it. 
5. Simple Fistulose Keratose Fibre. 
This form of fibre is usually very much larger and more rigid than the solid keratose 
fibre. It is cylindrical, and continuously fistular. The great central cavity of the fibre 
usually occupies about one-third of its diameter. It is nearly uniform in its size, but 
MDCCCLXII. 5 L 
