76 THE SEA-SIDE AND AQUARIUM. 



in that manner. On the other hand, however, it may 

 be observed, that the ' rasping' points on the surface 

 of the valves are never worn down, which must be 

 the case if they were constantly employed on hard 

 substances, as suggested; that in some of the species 

 the shells are externally quite smooth ; and that the 

 holes, instead of being quite cylindrical, are fitted to 

 the shells in such a manner as, at least when adult, to 

 deprive them of the power of lateral action. 



" The second theory, namely, that which endeavours 

 to account for the wearing away of the rock by the 

 rubbing of flinty atoms in the foot of the animal, has 

 this important difficulty to encounter j namely, that 

 after the most anxious microscopic researches, no such 

 silicious particles have been found in the Pholas, 

 although they do exist in some other families of boring 

 molluscs. 



"The third theory may be good as far as it goes, 

 but it is inadequate in itself to account for the whole 

 result. No doubt, currents of sea water between the 

 shell and the walls of its habitation most materially 

 aid, not only in clearing away the particles when 

 dislodged, but also to some extent in dislodging them. 



" As to the fourth, and perhaps on the whole the 

 most popular notion, that of a chemical solvent secreted 

 by the animal, it would probably puzzle the acutest 

 chemist to find a solvent which would act equally on 

 wood, clay, sandstone, chalk, and wax. 



" The fifth theory, or that of the combined action of 

 rasping and solution, combines the difficulty of both. 



