8 ANATOMY AND PHYSIOLOGY 



hard dry glue or glass would present to the eye. If these 

 spicula be now burned in the flame of a small spirit-lamp 

 until the combustion is completed and the mass is brought 

 to a white heat, and it be then examined as before, the 

 results are widely different in their aspect ; the spicula have 

 become considerably increased in diameter, and instead of 

 being solid, they are now extremely thin tubes of silex, lined 

 with a dense and nearly opake film of charcoal, rough and 

 granulated in its appearance. I thought in the first instance 

 that I might have unwittingly selected a fasciculus of young 

 spicula only, for burning, and I therefore repeated the 

 experiment, burning only half of the fasciculus and pre- 

 serving the remainder in an unaltered condition ; and on 

 carefully mounting the specimen in Canada balsam, I found 

 the same results precisely ; the unburned half of the 

 fasciculus presented all the characters of solidity that I 

 have before described, while the burned half was in perfect 

 unison with the previous results of incineration ; and at 

 the junction of the two, the transition from the one state to 

 the other might be readily traced even in single spicula. The 

 external coat of silex in these spicula is so thin and the coat 

 of charcoal with which it is lined so rough and opaque, that 

 the thickness of the silex cannot be readily ascertained ; 

 but in one of the short, stout, fusiformi-acerate spicula of 

 the dermal coat of the sponge, which is about the same 

 diameter as that of the skeleton spicula, I succeeded in 

 measuring the thickness of the siliceous coat accurately after 

 incineration. The length of the spiculum was ^th of an 

 inch, the greatest diameter ^th of an inch, and the thick- 

 ness of external siliceous case ^ c th of an inch. Figs. 

 251 and 252, Plate XI, represent portions of two of the 

 large spicula of the skeleton after incineration. 



I have very little doubt that the combustible matter in 

 the interior of these large spicula is really keratode, one of 

 the most elastic and durable animal substances with which 

 we are acquainted. The mode of its deposition within 

 these organs is precisely the same with that presented in 

 all the varieties of keratose fibre with which I am ac- 

 quainted ; and from its concentric arrangement, the nature 



