354 EVENINGS AT THE MICROSCOPE. 



The fleshy substance which surrounds these canals is of 

 a loose, spongy character, and grates beneath the knife : 

 a circumstance which is owing to the predominance of 

 the calcareous element here, as you will see when I ex- 

 tract a small portion of it, and, laying it on a slip of 

 glass, treat it with caustic potass. The microscope now 

 reveals a large number of spicula, far larger than those 

 we have hitherto observed, and different from either sort 

 in form. These resemble very gnarled branches of oak, 

 with the branchlets broken off close to their origin, leaving 

 ragged and starred ends. 



