OF THE SPONGIAD^E. 75 



4. Multi-spiculaled 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 

 exceedingly 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. 264, Plate XIII, represents a fibre from the 

 skeleton of Desmatidon agayropila, Bowerbank. 



5. Inequi-spiculated Keratose Fibre. 



This form of fibre is composed of an infinite number of 

 spicula disposed in every possible direction, 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, Bapltyrus Griffithsii, 

 Bowerbank, the fibre is very unequal in size and much 

 varied in its form, frequently becoming very much flattened 

 and expanded. Fig. 265, Plate XIII, represents a longi- 

 tudinal section of a small portion of a fibre from the 

 skeleton, showing the irregular disposition of the spicula 

 within it. 



6. 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 occasionally it is dilated 

 considerably for a short space, and then resumes its original 



