PERILS OF THE OYSTER. 237 



Hancock remarks : " The extreme simplicity of the 

 organic structure of these beings forbids a belief in the 

 existence of a special secreting apparatus. If, therefore, a 

 solvent fluid be the agent, it must be supposed to exude 

 from the entire surface of this humble animal. The 

 character of the excavations would also lead to the same 

 conclusion ; for it is evident that the form of the sponge 

 is influential in determining that of the chambers it inha- 

 bits. The test, then, can be easily applied, and, were the 

 secretion of an acid nature, there could be little difficulty, 

 one would think, in detecting it, particularly as cliona 

 appears to work perpetually, at least so long as it continues 

 to grow. I have completely failed, however, in detecting 

 an acid." The calcareous particles, which may generally 

 be observed strewed along the branched channels in the 

 'shell of the oyster, when inhabited by cliona, afford, as 

 Mr. Hancock observes, a pretty strong proof of mechanical 

 agency. 



This, however, he proceeds, I think satisfactorily, to 

 describe ; for, if a portion of the superficial covering of the 

 animal be carefully removed, and placed between plates of 

 glass, with the external surface uppermost, and treated with 

 strong nitric acid, large crystalline bodies of a peculiar 

 character are scattered over it. These bodies are of a pale 

 straw colour, and of the most brilliant lustre and gem-like 

 beauty, the largest measuring one six-hundredth of an inch 

 across. They are mostly irregularly six-sided, depressed, 

 and scale-like, but stout, and frequently thickened in the 

 centre, the upper surface being covered with numerous 

 elevated, lozenge-shaped points, each generally having an 

 expanded base, of a squarish form, slightly raised above 

 the common surface. 



