Lastly, Mr. Harper presented the Editor of the "Athenæum" with a piece of bored rock, of which he has several specimens. He adds, "On examination, you will perceive that the larger Pholas must have bored through its smaller and weaker neighbour (how suggestive!), the shell of the latter, most fortunately, remaining in its own cavity."
Now, Mr. Robertson claimed for his observation of this phenomenon novelty and originality; but Mr. Harper stoutly maintained it to be "as common to the eye of the practised geologist as rain or sunshine." The details are curious; though some impatient, and not very grateful reader, may imagine himself in the condition of the shell of the smaller Pholas, and will be, as he deserves to remain, in the minority. [29]
It may be interesting to sum up a few of the opinions of the mode by which these boring operations are performed. Professor Forbes states the mode by which Molluscs bore into wood and other materials is as follows:—"Some of the Gauterspods have tongues covered with silica to enable them to bore, and it was probably by some process of this kind that all the Molluscs bored."
Mr. Peach never observed the species of Pholas to turn round in their holes, as stated by some observers, although he had watched them with great attention. Mr. Charlesworth refers to the fact that, in one species of shell, not only does the hole in the rock which the animal occupies increase in size, but also the hole through which it projects its syphons.
Professor John Phillips, alluding to the theories which have been given of the mode in which Molluscs bore into the rocks in which they live, believes that an exclusively mechanical theory will not account for the phenomenon; and he is inclined to adopt the view of Dr. T. Williams—that the boring of the Pholades can only be explained on the principle which involves a chemical as well as a mechanical agency.
Mr. E. Ray Lankester notices that the boring of Annelids seems quite unknown; and he mentions two cases, one by a worm called Leucadore, the other by a Sabella. Leucadore is very abundant on some shores, where boulders and pebbles may be found worm-eaten and riddled by them. Only stones composed of carbonate of lime are bored by them. On coasts where such stones are rare, they are selected, and others are left. The worms are quite soft, and armed only with horny bristles. How, then, do they bore? Mr. Lankester maintains that it is by carbonic acid and other acid excretions of their bodies, aided by the mechanical action of their bristles. The selection of a material soluble in these acids is most noticeable, since the softest chalk and the hardest limestone are bored with the same facility. This can only be by chemical action. If, then, we have a case of chemical boring in these worms, is it not probable that many Molluscs are similarly assisted in their excavations?
FOOTNOTES:
[27] How Brunel took his construction of the Thames Tunnel from observing the bore of the Teredo navalis in the keel of a ship, in 1814, is well known.
[28] "Athenæum," No. 1640.
[29] See also "Life in the Sea," in "Strange Stories of the Animal World," by the author of the present volume. Second Edition. 1868.