72 ANATOMY AND PHYSIOLOGY 



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 in its natural con- 

 dition, notwithstanding there is frequently an internal axis 

 of extraneous matter or of spicula, it is often remarkably 

 soft and flexible. The spicula, although immersed in the 

 fibre, evidently possess a considerable 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 semi-transparent. 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 nine typical forms : 



1. Solid simple keratose fibre. 



2. Spiculated keratose fibre. 



3. Hetro-spiculated keratose fibre. 



4. Multi-spiculated keratose fibre. 



5. Inequi-spiculated keratose fibre. 



6. Simple fistulose keratose fibre. 



7. Compound fistulose keratose fibre. 



8. Regular arenated keratose fibre. 



9. Irregularly arenated keratose fibre. 



1 . /Solid Simple Keratose Fibre. 



The typical form of this description of fibre is that which 

 forms the skeleton of the Turkey sponges of commerce, the 

 structure of which I described in a paper read before the 

 Microscopical Society of London, and published in vol. i, 

 p. 42, of its ' Transactions.' The mature fibre is perfectly 

 solid, and no vestige of a central cavity can be observed in 

 any part of it, either when viewed by transmitted light or 



