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. 2d, That the combined action of the secreted solvent, and rasping by the valves, effects the perforations. 3d, That the holes are made by rasping 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 ingenious, 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 rasping and the secreted solvent, is, for obvious reasons, equally objectionable.
The third theory, which endeavours to account for the wearing away of the rock by means of silicious particles situated in the foot and other parts of the animal, has been for some time proved to be erroneous, from the fact, that the combined skill of some of our best anatomists and microscopists has failed to discover the slightest presence of any particles of silex in the Pholadidæ, although these are believed to exist in other families of the boring acephala.
The fourth theory, that of ciliary currents as an accessory agent in boring, is worthy of greater consideration, chiefly from the evidence we possess of the immense power which the incessant action of currents of water possess in wearing away hard substances.
We come now to what may be considered the most important of the theories above enumerated, viz., the mechanical action of the valves of the Pholas in rasping away the rock, &c. This hypothesis is one which most naturally suggests itself to the mind of any impartial person, on examining, for instance, the rasp-like exterior of the shell of Pholas crispata.[12] But as I shall endeavour to show, although the shell forms the principal, it does not by any means constitute the sole agent in completing the perforating process.
Mr. Clark, a clever naturalist, considers with Mr. Hancock that the powerfully armed ventral portion of the mantle of the closed boring acephala is fully adequate to rub down their habitations, and that the theories of mechanical boring, solvents, and ciliary currents, are so utterly worthless and incapable of producing the effects assigned to them, as not to be worth dwelling upon for one moment. Mr. Clark, therefore, comes to the conclusion that 'the foot is the true and sole terebrating agent in the Pholas.' This 'fact' he considers to be 'incontestably proved,' for the following reason, viz., because he had discovered specimens of this bivalve with the foot entirely obliterated,—which phenomenon, Mr. Clark states, is caused by the animal having arrived at its full growth, at which period the terebrating functions cease; and as 'nature never permanently retains what is superfluous,' the foot is supposed gradually to wither away, and finally disappear.
This, I suspect, is another 'fancy' theory. Although I have excavated hundreds of Pholades, some of giant-like proportions, it has never been my lot to witness the foot otherwise than in a healthy and fully developed condition.