'But,' you ask, 'what then was the opal stick?' Ay, there's the rub, for even the greatest naturalists cannot positively agree as to the use and purpose of this mysterious organ. Yet it is to be found (as we have seen) in the foot of the Pholas, in the Mussel, the Cockle; and, in fact, it occurs in almost all bivalves both great and small. It is termed the 'hyaline cylindrical stylet,' and is very _lucidly_ and scientifically described as 'an elastic spring to work the corneous plate or attritor, and by the muscular action of the foot and body, to divide and comminute the food, and especially the minute crustaceous and testaceous alimentary matters received into the stomachial cavity. It appears then that this appendage acts as _a gizzard_, and the bivalve mollusca are thus supplied with a masticatory apparatus very analogous to the gizzards of some of the gasteropoda.' Now, the simple fact that I have stated above, of the hyaline stylet being found in the _foot_, and not in the stomach, at once proves that it cannot possibly act as a _gizzard_ to the Pholas, or any other bivalve in which it is known to exist. In the succeeding chapter I shall endeavour, from personal observation, to shed a slender ray of light upon the function of the stylet. CHAPTER XVI. The Pholas, &c. (ROCK-BORERS.) 'He that of greatest works is finisher Oft does them by the meanest minister.' XVI. At certain parts of the Scottish coast, the 'dykes,' or walls built near the road-side, are constructed entirely of rough-hewn pieces of hard sandstone rock, brought from the neighbouring shore. Sometimes a dyke will extend for two or three miles, without presenting an isolated fragment, in which the honeycomb-like perforations of certain species of the boring Mollusca are not more or less apparent. A fragment of soft sandstone lies before me, measuring three and a half inches in length, and two inches in breadth, which, small though it be, contains no fewer than seventeen cylindrical tunnels. Each of these exhibits so wonderful a skill in construction, that human hands could not surpass it, though aided by 'all the means and appliances to boot,' of mechanical agency. It is generally stated that the Pholas never intrudes itself into the apartment occupied by a neighbouring 'worker.' The Pholas, however, often intrudes on its neighbour; and such intrusion is manifested in the small piece of stone alluded to in no less than four instances. Here let me observe, that it is not always the larger mollusc that bores through the smaller one; it as frequently happens that the latter deserves the brand of wanton aggressor. Both cases are common enough, and, indeed, must of necessity occur, wherever at any time a colony of various sized Pholades are clustered together in a small portion of rock.