VARIOUS THEORIES ON THE SUBJECT. 249 



The various theories promulgated on this knotty 

 point are generally classed under five heads : 1st, 

 That the animal secretes a chemical solvent an acid 

 which dissolves the substance in which it bores. 

 zd, That the combined action of the secreted sol- 

 vent, and rasping by the valves, effects the per- 

 forations. 3d, That the holes are made by rasp- 

 ing effected by silicious particles studding the 

 substance of certain parts of the animal. 4th, 

 That currents of water, set in action by the 

 motions of vibratile cilia, are the agents. 5th, and 

 lastly, That the boring mollusca perforate by means 

 of the rotation of their shells, which serve as 

 augurs. 



Of all the above, the first which is quite a fancy 

 theory, seems to meet with greatest favour among 

 certain naturalists. But as it is rather puzzling to 

 find a chemical solvent, which will act equally upon 

 sandstone, clay, chalk, wax, and wood, this hypothesis 

 can only be looked upon by practical men as in- 

 genious, but incorrect. Even were it proved that 

 the animal really possessed the power of secreting 

 an acid sufficiently powerful, the question naturally 

 arises, How can the shell escape being affected in 

 like manner with the much harder substance in 

 which it is situated ? 



The second theory, or the combined action of rasp- 

 ing and the secreted solvent, is, for obvious reasons, 

 equally objectionable, 



