OF THE SPONGIAD^. 79 



the manner of the secretion of pure keratode in the fibres 

 of the sponges of commerce. When cleansed from the 

 sarcodous matter by which they are surrounded in a living 

 state, the fibrous skeleton bears a striking resemblance to 

 fibres of spun glass, and is quite as pellucid and colourless 

 as the aiiificial material, and the dead sponge quite as 

 brittle. The fibrous skeleton of Bactylocalyx pumicea, 

 Stutchbury, in its mode of arrangement strikingly resembles 

 that of one of the sponges of commerce ; it is equally com- 

 plex and irregular in its structure, and the component 

 fibres quite as much anastomosed. In that species the 

 fibres are smooth and cylindrical, but in others they 

 frequently abound with minute, obtuse, wart-like elevations,. 



There is every indication in the skeletons that the increase 

 in diameter, and the extension in length in the fibres, is 

 efiected in the same manner as in the solid keratose fibres. 

 The free terminations of the young fibres have the same 

 attenuated but obtuse form, and the pullulation of the 

 young fibres from the sides of the mature ones is quite as 

 apparent as in their keratose congeners, but they never 

 appear to be in the young state, as the keratose ones fre- 

 quently are, viscid ; and extraneous matters are never 

 detected at their apices, or on their substance. 



There are two distinct forms of this class of fibre : 



1st. Solid sihceous fibre. 



2nd. Simple fistulose siliceous fibre. 



The structure of solid siliceous fibre is very similar to 

 that of solid keratose fibre. Occasionally there are indica- 

 tions of a former existence of a minute central canal ; but 

 in the fully developed fibre this is rarely visible. The 

 external characters of these fibres vary in each species. In 

 a new siliceous sponge in the British Museum, designated 

 by Dr. Gray M'Andrewsia azoiea, the fibres are quite 

 smooth, as represented by Fig. 374, Plate XV. But in 

 the greater number of species they are more or less tuber- 

 culated, as in Fig. 275, Plate XV, which represents a group 

 of fibres from the type-specimen of Bactylocalyx pmmcea, 

 Stutchbury, a portion of which is in the possession of Dr. 

 J. E. Gray. In other species in my possession the tuber- 



