8 THE BRITISH SPONGES. 



comparatively free. These sponges have been compared to 



the crumb of bread, and the comparison conveys a very 



Fig.X 



exact idea of their structm*e as seen with the naked eye. 

 In a third class of sponges ( Grantia) there is no net-work, 

 their basis being a porous membrane rendered compact 

 and firm by the profusion of spicula immersed in it, and 

 these consist solely of carbonate of lime. The siliceous 

 spicula form mostly needle-like spines, but there are found 

 along with them, in the genus Tethya, some that might 

 have been the model from wliich mythological painters 

 have drawn the trident they have placed in the hands of 

 Neptune. (Fig. 4.) The calcareous spicula are more va- 

 riously shaped, — either simple and acicular, or clavate, or 

 formed with three or even, sometimes, with four prongs. 

 (Fig. 5.) The two kinds, viz. the calcareous and sili- 

 ceous, have not hitherto been detected coexistent in any 

 native sponge ; but the spicula of every species are very 

 constant to the same figure, although in point of size they 

 vary exceedingly, nor have I been able to discover anv 



