OF THE SPONGIADiE. 73 



in transverse sections of the fibre, by the aid of a Lieber- 

 kuhn. Occasionally, but very rarely, I have seen in young 

 and immature fibres faint and irregular indications of there 

 having been a very small central cavity in perhaps the 

 earliest period of its development, but in the mature fibre 

 I have never been able to trace such cavities (Fig. 261, 

 Plate XIII). 



This description of fibre is occasionally surrounded by a 

 membranous sheath, on vphich is imbedded a beautiful 

 system of hollow fibrils or vessels, which sometimes wind 

 round the skeleton-fibre in a spiral direction ; at others it 

 assumes a longitudinal course, giving ofi" short csecoid 

 branches; or it forms a complex and irregular network. 

 In an Australian sponge in my possession, the latter mode 

 is the only form in which it occurs. In some of these 

 minute fibrils or vessels I observed numerous minute 

 globules, which were rendered movable by a slight pres- 

 sure on the glass under which they were exhibited. The 

 mean diameter of these tubes or vessels was g-j^ inch. This 

 tissue is of rare occurrence, and I have been unable to de- 

 termine whether it is a specific character, or whether it is 

 due to a peculiar condition of the sponge. Pig. 279, 

 Plate XVI, represents a portion of fibre from the skeleton of 

 one of the sponges of commerce. Pig. 380, Plate XVI, is 

 from a rigid species of Australian sponge. This singular 

 tissue is described more fully in a paper which I read 

 before the Microscopical Society of London in 1841, and 

 which is published in their ' Transactions,' vol. i, p. 32, 

 plate iii. 



2. Spiculated Keratose Fibre. 



This structure is essentially a solid form of keratose 

 fibre, no central cavity ever being visible in its axis. The 

 normal form of the fibre is cylindrical, but it is occasionally 

 more or less compressed, and always contains a thin central 

 line or axis of spicula arranged in longitudinal series. The 

 spicula are secreted within the fibre, and are nearly uniform 

 in size, and always of the same shape in the same species 



