WERE a young naturalist asked to exemplify what man has learned from the lower animals, he could scarcely adduce a more striking instance than that of a submarine shelly worker teaching him how to execute some of his noblest works. This we have learned from the life and labours of the Pholas, of which it has been emphatically said:—"Numerous accounts have been published during the last fourteen years in every civilized country and language of the boring process of the Pholas; and machines formed on the model of its mechanism have for years been tunnelling Mont Cenis."

In the Eastern Zoological Gallery of the British Museum, cases 35 and 36, as well as in the Museum of Economic Geology in Piccadilly, may be seen specimens of the above very curious order of Conchifers, most of the members of which are distinguished by their habits of boring or digging, a process in which they are assisted by the peculiar formation of the foot, from which they derive their name. Of these ten families one of the most characteristic is that of the Razor-shells, which, when the valves are shut, are of a long, flattened, cylindrical shape, and open at both ends. Projecting its strong pointed foot at one of these ends, the solen can work itself down into the sand with great rapidity, while at the upper end its respiratory tubes are shot out to bring the water to its gills. Of the PholadÆ, the shells of which are sometimes called multivalve, because, in addition to the two chief portions, they have a number of smaller accessory pieces, some bore in hard mud, others in wood, and others in rocks. They fix themselves firmly by the powerful foot, and then make the shell revolve; the sharp edges of this commence the perforation, which is afterwards enlarged by the rasp-like action of the rough exterior; and though the shell must be constantly worn down, yet it is replaced by a new formation from the animal, so as never to be unfit for its purpose. The typical bivalve of this family is the Pholas, which bores into limestone-rock and other hard material, and commits ravages on the piers, breakwaters, &c., that it selects for a home.

In the same family as the above Dr. Gray ranks the Teredo, [27] or wood-boring mollusc, whose ravages on ships, piles, wooden piers, &c., at sea resemble those of the white ant on furniture, joints of houses, &c., on shore. Perforating the timber by exactly the same process as that by which the Pholas perforates the stones, the Teredo advances continually, eating out a contorted tube or gallery, which it lines behind it with calcareous matter, and through which it continues to breathe the water.

The priority of the demonstration of the Pholas and its "boring habits" has been much disputed. The evidence is full of curious details. It appears that Mr. Harper, of Edinburgh, author of "The Sea-side and Aquarium," having claimed the lead. Mr. Robertson, of Brighton, writes to dispute the originality; adding that he publicly exhibited Pholades in the Pavilion at Brighton in July, 1851, perforating chalk rocks by the raspings of their valves and squirtings of their syphons. Professor Flourens (says Mr. Robertson) taught my observations to his class in Paris in 1853; I published them in 1851, and again more fully in the "Journal de Conchyliologie," in 1853; and M. Emile Blanchard illustrated them in the same year in his "Organisation du RÈgne Animal." I published a popular account of the perforating processes in "Household Words" in 1856. After obtaining the suffrages of the French authorities, I have been recently honoured with those of the British naturalist. (See Woodward's "Recent and Fossil Shells," p. 327. Family, PholadidÆ.) On returning to England last autumn I exhibited perforating Pholades to all the naturalists who cared to watch them. An intelligent lady whom I supplied with Pholades has made a really new and original observation, which I may take this opportunity of communicating to the public. She observed two Pholades whose perforations were bringing them nearer and nearer to each other. Their mutual raspings were wearing away the thin partition which separated their crypts. She was curious to know what they would do when they met, and watched them closely. When the two perforating shell-fish met and found themselves in each other's way, the stronger just bored right through the weaker Pholas. [28]

Mr. Robertson has communicated to "Jameson's Journal," No. 101, the results of his opportunities of studying the Pholas, during six months, to discover how this mollusc makes its hole or crypt in the chalk: by a chemical solvent? by absorption? by ciliary currents? or by rotatory motions? Between twenty and thirty of these creatures were at work in lumps of chalk, in sea-water, in a finger-glass, and open for three months; and by watching their operations. Mr. Robertson became convinced that the Pholas makes its hole by grating the chalk with its rasp-like valves, licking it up when pulverized with its foot, forcing it up through its principal orbrambial syphon, and squirting it out in oblong nodules. The crypt protects the Pholas from confervÆ, which, when they get at it, grow not merely outside, but even with the lips of the valves, preventing the action of the syphons. In the foot there is a gelatinous spring or style, which, even when taken out, has great elasticity, and which seems the mainspring of the motions of the Pholas.

Upon this Dr. James Stark, of Edinburgh, writes:

—"Mr. Robertson, of Brighton, claims the merit of teaching that Pholades perforate rocks by 'the rasping of their valves and the squirting of their syphons.' His observations only appear to reach back to 1851. But the late Mr. John Stark, of Edinburgh, author of the 'Elements of Natural History,' read a paper before the Royal Society of Edinburgh, in 1826, which was printed in the Society's 'Transactions' of that year, in which he demonstrated that the Pholades perforate the shale rocks in which they occur on this coast, by means of the rasping of their valves, and not by acids or other secretions. From also finding that their shells scratched limestone without injury to the fine rasping rugosities, he inferred that it was by the same agency they perforated the hard limestone rocks."

To this Mr. Robertson replies, that Mr. Osler also, in 1826, demonstrated that the Pholades "perforate the shale rocks by means of the rasping of their valves; and more, for he actually witnessed a rotatory movement. But RÉaumur and Poli had done as much as this in the eighteenth and Sibbald in the seventeenth century: and yet I found the solvent hypothesis in the ascendant among naturalists in 1835, when I first interested myself in the controversy. What I did in 1851 was, I exhibited Pholades at work perforating rocks, and explained how they did it. What I have done is, I have made future controversy impossible, by exhibiting the animals at work, and by discovering the anatomy and the physiology of the perforating instruments. In the words of M. Flourens, 'I made the animals work before my eyes,' and I 'made known their mechanism.' The discovery of the function of the hyaline stylet is not merely a new discovery, it is the discovery of a kind of instrument as yet unique in physiology."

Mr. Harper having termed the boring organ of the Pholas the "hyaline stylet," found it to have puzzled some of the disputants, whereupon Mr. Harper writes:—"Its use up to the present time has been a mystery, but the general opinion of authors seems to be, that it is the gizzard of the Pholas. This I very much doubt, for it is my belief that the presence of such an important muscle is solely for the purpose of aiding the animal's boring operations. Being situated in the centre of the foot, we can readily conceive the great increase of strength thus conveyed to the latter member, which is made to act as a powerful fulcrum, by the exercise of which the animal rotates—and at the same time presses its shell against and rasps the surface of the rock. The question being asked, 'How can the stylet be procured to satisfy curiosity?' I answer, by adopting the following extremely simple plan. Having disentombed a specimen, with the point of a sharp instrument cut a slit in the base of its foot, and the object of your search will be distinctly visible in the shape of, if I may so term it, an opal cylinder. Sometimes I have seen the point of this organ spring out beyond the incision, made as above described."

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?

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