HYPOTHESES OF THEIR FORMATION. 55 



while, confessedly, no such acid had ever been detected 

 by the most careful tests. 



Others maintained that the rough points which stud 

 the shell enable it to serve as a rasp, which the animal, 

 by rotating on its axis, uses to wear away the stone or 

 other material ; but it was difficult to understand how 

 it was that the shell itself was not worn away in the 

 abrasion. 



Another zoologist, rejecting this hypothesis, main- 

 tained that the edges of the mantle and the short thick 

 foot are the instruments employed ; and that, though 

 these fleshy organs seem little fitted for such work, 

 they are really endowed with the requisite power in 

 the shape of crystals of flint which are deposited thickly 

 in their substance. Strange to say, however, other 

 accurate observers fail to detect these siliceous crystals, 

 and therefore reject the hypothesis. 



Another suggested that the stone was removed in in- 

 visible particles by the constant action of currents in 

 the water, produced by vibratile cilia seated on the soft 

 parts ' of the animal ; but this supposition was found 

 untenable on examination. 



Actual observation in the aquarium has proved that 

 the second hypothesis is the true one. M. Cailliaud in 

 France and Mr. Eobertson in England have demon- 

 strated that the Pholas uses its -shell as a rasp, wearing 



