THE BRITISH SPONGES. 9 



certain order in their arrangement. They are proba- 

 bly hollow in the centre and closed at both ends. " When 

 the spicula are examined through the microscope after this 

 Fig. 4. Fig. 5. 



cxposm'e to heat, we distinctly perceive," says Dr Grant, 

 " a shut cavity^ within them, extending from the one point 

 to the other ; and on the inflated part of each spiculum we 

 observe a ragged opening, as if a portion had been driven 

 out by the expansion of some contained fluid. In those 

 spicula which had suffered little change of form by their in- 

 candescence, I have never failed to observe the same cavity 

 within, extending from one end to the other, and a distinct 

 open rent on their side, by wliich the contained matter has 

 escaped before the usual globular distension had taken 

 place."* The sponges containing spicula, in any degree of 

 profusion, are inapplicable to domestic purposes. 



Sponges are all aquatic productions. " In their natural 

 state they are soft and elastic, and possess lively colours ; 

 but many of the species, by di-ying, become quite friable, 

 lose their fine shades of colour, and become white. Soon 



• Edin. Phil. Journ. xiv. p. 184 — According to Mr Bovverbank, the 

 spicula have a central cavity " lined with an aninnal membrane, which 

 becomes converted into a thin film of carbon when the spicula are ex- 

 posed to the action of the blow-pipe " — Ann. Nat. Hist. vii. p. 74. 



