238 OYSTERS, AND ALL ABOUT THEM. 



These bodies are frequently congregated into groups, 

 and are occasionally placed together side by side. Strong 

 nitric acid does not in the least affect them after many 

 days' immersion, the sharp angularity of the elevated 

 points remaining unimpaired, and their brilliancy undi- 

 minished. From these facts, and from the manner in 

 which these bodies reflect light, there can be little doubt 

 that they are composed of silex. or of some other substance 

 equally dense. 



Nor is this all ; other crystalline bodies crowd the 

 surface, which are as brilliant as the former, and, like 

 them, resist strong nitric acid. These are mostly minute, 

 being generally one six-thousandth of an inch wide ; they 

 vary, however, considerably in size, and are occasionally 

 very much larger ; they are mostly angulated, having an 

 expanded scale-like base, and much resemble the lozenge- 

 shaped points of the larger bodies. These smaller ones 

 are crowded together into dense masses ; occasionally they 

 become united by their expanded bases, and then the mass 

 has a considerable resemblance to the larger forms. 



All, then, that is required, is that each granule, or 

 cluster of granules, should be set in motion ; nor would 

 anything more than limited action be necessary. The 

 motion of cilia, according to Ehrenberg, is produced by 

 the contractile tissue on which they are based ; and if the 

 silicious bodies now described are connected with a similar 

 tissue, the whole surface of the sponge would be composed 

 of thousands of minute drills, quite able to cut into the 

 hardest calcareous substances. (/;) 



Amongst birds, besides the oyster-catcher, the Royston 

 crow and the carrion crow have the character of destroying 

 oysters. 



(h) "Annals of Nat. Hist.," 2nd series, vol. 3, pp. 321-348. 



