BRITISH SPONGIAD.E. 383 



together; in others, the fibres, although thickly covered 

 with patches of sand, are still separate and distinct; in 

 other specimens the fibres are cylindrical and symmetrica], 

 with included grains, but with few external ones ; and the 

 last two modes may be frequently observed to occur on the 

 opposite side of the same specimen, so that the amount of 

 the arenation of the fibre is evidently dependent on the 

 peculiarities of its locality, and the paucity or abundance of 

 sand or other extraneous matters suited to its purpose, that 

 is found in its neighbourhood. Thus, in the specimens 

 found on the shore at Brighton, where little sand compara- 

 tively is found, the adventitious substance incorporated on 

 the skeleton abounds with various forms of sponge spicula, 

 while those from the neighbourhood of the sands of Hast- 

 ings are filled with sandy particles, and but rarely exhibit 

 specimens of spicula; and I am therefore led to believe that 

 the latter, when they do occur in the sponge, are truly ad- 

 ventitious and not secreted by the animal under any cir- 

 cumstances. In the living state the sarcode is very abundant, 

 filling up nearly the whole of the interstitial spaces, and 

 causing the sponge to have a firm and fleshy texture. 



In some living specimens obtained at Brighton, I 

 observed, besides the arenaceous and solid keratose fibres, 

 that there were occasionally others that were fistulous, 

 sometimes containing a few grains of sand and spicula, more 

 or less numerous. The tubular fibres are smaller than the 

 arenaceous ones ; they originate from them, and anastomose 

 freely with them ; the fistulous cavity is very irregular in 

 its size, sometimes being large and continuous, or suddenly 

 contracting and becoming small in diameter. These fibres 

 appear to be destined to fill an important office in the 

 economy of the animal for in many of the Brighton 

 specimens I observed gemmules of various sizes imbedded 

 in the parietes of the tubes, some of them projecting from 

 the inner surface towards the centre of the fistulous cavity ; 

 but the greater number were projected from the outer 

 surface of the fibre in all degrees short of absolute separa- 

 tion. When view r ed with a power of 300 linear, they are 

 seen to have a large opaque nucleus, of a somewhat 



