Popular Science Monthly/Volume 3/May 1873/The Borers of the Sea
MANY stories are current as to how inventors have borrowed or stolen their ideas from Nature, and there has been much ingenious discussion as to whether hints thus appropriated are properly patentable. Boring is an example of natural processes that have been thus used by art, and it is remarkable that the lowest creatures are the most skilful mechanics in this particular. An eminent living inventor, who has made a fortune out of a patent auger, hit upon the method followed by the most successful insects which bore into hard wood. And so we are assured that the celebrated engineer Brunei, in constructing the Thames Tunnel, but imitated the shell-lined burrow of the Teredo navalis, or Ship-worm. This mollusk in shape resembles a worm, and surrounds itself with a shell open at both ends. From the mouth it can protrude its short foot, and the other extremity of its body; the "tail" is bifurcated, one prong being the inspirator and the other the expirator tube of the siphon which constitutes the animal's nutritive apparatus.
It has long been a subject of controversy among naturalists how the Ship-worm and other mollusks of the same family bore their way into the rocks and timbers which they penetrate. As regards the Pholades, for instance, Mr. Robertson, who kept these animals alive in their chalky burrows, and studied their habits with the closest attention, found that when burrowing they make a half-revolution of their shell to the right, and then back to the left, after the manner of a carpenter using a brad-awl. The Pholas is a bivalve, club-shaped, and the outer surface of its shell is covered with small teeth in curves, and resembling the face of a rasp. These teeth would naturally seem well suited for the purpose of boring, yet all naturalists are not agreed on this point. Thus, some hold that the animal secretes an acid solvent, which causes the material in which it is burrowing to decay. Then only is it that, securing itself with its sucker-like foot, it works itself from right to left, and vice versa, to widen the passage. But Mr. Gwynn Jeffreys, as stated in the December number of The Popular Science Monthly, is of opinion that the foot, which he says is charged with siliceous particles, is the true boring apparatus of all the conchifera, and acts like the leaden wheel of the lapidary.
The history of the development of the Teredo is thus given by M. de Quatrefages: "The larva, which is at first almost spherical and entirely covered with vibratile cilia, may be compared to a very minute hedgehog, in which every spine acts as a natatory organ. It swims in all directions with extreme agility, and this first state continues about a day and a half. Toward the end of this time the external skin bursts, and, after being incrusted with calcareous salts, becomes a shell, which is at first oval, then triangular, and at last very nearly spherical. While the shell is being formed, the vibratile cilia disappear, but the little animal is not on that account condemned to inactivity. In proportion as the external cilia diminish, we observe that another equally ciliated organ becomes developed, which widens and extends in such a manner as to form a large collar or ruff margined with fringes. This new organ of locomotion may be entirely concealed within the shell, or may be extended from it, and acts in the manner of the paddle-wheel of a steamboat.
"By means of this apparatus the young larva continues to swim with as much facility as in its earlier age; but it now, moreover, acquires another organ, a sort of fleshy foot, which can be extended and contracted at will. It has also organs of hearing similar to those of several mollusks, and eyes analogous to those of certain annelids." The last metamorphosis is when the Teredo takes its worm-shape, and is ready to commence its boring operations.
The Teredo is supposed to have been originally a native of tropical or semitropical seas, though now it is found in high latitudes. It does not appear to have been known to the Greeks or Romans, or at least its ravages in ancient times could not have been very great, else the unsheathed hulls of Greek and Roman vessels would have been perforated. The Pholas penetrates stone as well as wood, but the Teredo loves most to burrow into timber.
The damage done to submerged timbers by the Teredo is enormous. It once threatened the dikes of Holland with destruction. A portion of the pier at Yarmouth was so honey-combed with perforations that it might easily be crushed between the hands as though it were paper, the partition between the various tubes being in many places as thin as parchment. A piece broken off this pier, and measuring about 7 by 11 inches, weighed less than four ounces, including the shelly lining of the tubes. In the space of 40 days a piece of deal was fairly riddled by these borers, and Wood, in his "Natural History," gives an instance of their attacking a floating block of oak. This block had a large iron bolt passing through its centre, the rusting of which preserved the timber for a small space all around from the attacks of the borers. But all the block not so protected was honey-combed.
The Ship-worm always makes its perforations in the direction of the grain of the timber, except where a knot, or the shell of another Teredo, or hindrance of any kind is met with, and then it takes a turn according to circumstances. The animal begins to bore long before it has reached its full size, and it grows within the cavity which it makes. When taken out of the tube the Ship-worm is found to be a long, grayish-white animal, about one foot long and half an inch thick, with rounded head and forked tail. The Giant Teredo of Sumatra attains the length of six feet, and a diameter of three inches. This animal, however, differs from the Ship-worm in this, that it does not penetrate timber, but only burrows into the hardened mud of the sea-bed.
The use of copper-sheathing to protect ships from the Ship-worm is so well known that it need but be simply referred to here. It is not perhaps so generally known that, if timber be driven full of iron nails, the same object is attained. Another method of protecting woodwork consists in forcing into its pores a solution of corrosive sublimate. The only objection to this method is its great cost. Quatrefages, however, asserts that one twenty-millionth part of corrosive sublimate is enough to destroy all the young Ship-worms in two hours. He, therefore, proposes that ships should be cleared of this fearful pest by being taken into a closed dock, into which a few handfuls of corrosive sublimate should be thrown and well mixed with the water. The salts of copper and lead have a similar effect, but do not act so instantaneously.
The Teredo does not perforate rock, but the Pholas acts an important part in bringing about geological changes, owing to his habit of boring rocks. There is no doubt that the chalk-cliffs of England are first tunnelled by the Pholades, and then gradually destroyed by the waves of the sea.
Of the Date-shell, another very interesting borer, Wood gives the following account: "It is truly a wonderful little shell. Some of the hardest stones and stoutest shells are found pierced by hundreds of these curious beings, which seem to have one prevailing instinct, namely, to bore their way through every thing. Onward, ever onward, seems to be the law of their existence, and most thoroughly do they carry it out. They care little for obstacles, and, if one of their own kind happens to cross their path, they quietly proceed with their work, and drive their tunnel completely through the body of their companion."
Of the Saxicava rugosa, another borer, Wood gives this description: "It is a flattish bivalve, symmetrical in shape when young, but oblong when old. It burrows as rapidly as the Lithodomus, and into rock of adamantine density. Sometimes it bores into corals, frequently into limestone, and often into shells, which it penetrates as deeply as the Date-shell. Some of the enormous stones employed in building the Plymouth Breakwater are now much wasted by the holes made in them by the Saxicava." Like the Date-shell, too, this animal runs its tunnels at every angle, and turns out of its course for no consideration whatever.
The Razor-shell makes a burrow in the sand, and there lives with its siphon, or recurved food-tube, appearing just above the mouth of the burrow. It may often be seen "spouting," or sending forth small jets of water from its hiding-place in the sand after the tide has retreated. On examining the spot cautiously—for the creature is somewhat shy—two round holes in the sand, answering to the two fringed openings of the Razor-shell's siphon, will be seen, resembling a key-hole, and each large enough to admit a common goose-quill. But, if the animal be approached rudely, or if the finger be placed on the openings, the mollusk disappears deep in the burrow. The Razor-shell is possessed of a very muscular "foot," as it is called, but it might as well be named a hand or a tongue. By means of this organ, which they elongate or contract at pleasure, the animals are enabled to burrow and to go "up and down stairs" with great rapidity. It requires dexterous management to capture the Razor-shell alive. When they are wanted for food or for bait, the usual plan is to shoot into the sand, alongside of a "spout," a hooked iron rod, which must be at once pulled out again obliquely, so as to fetch the shell.
A better way is to drop a little salt on its tail, or at least on its siphon-orifices. If this be done, the animal will rise partly out of its burrow—for it hates undiluted chloride of sodium—and may then be captured, if you be quick. But, if you should fail to seize the creature at the first attempt, in vain would you pour salt in the burrow; the mollusk now sees the artifice, and is not to be imposed upon a second time.
The Aspergillum, or Watering-pot Shell, derives its name from its perforated disk, which much resembles the snout of a watering-pot. This animal burrows into sand or bores into stone, wood, or thick shells.
When in its burrow, its narrow end, containing the openings of its siphon, protrudes. To the same group belongs the Flask-shell, which perforates shells of every kind, attaching them to itself by means of some natural cement. It thus often constructs around itself a casing like a flask, and hence its name.
We will close this notice of the Borers of the Sea with some account of the Mya arenaria, or Gaper-shell, which burrows into sand, and which derives its name, gaper, from the fact that its bivalve-shell gapes, to allow its long, stout tube to protrude. "It inhabits sandy and muddy shores," says Wood, "and, to an inexperienced eye, is quite invisible. The shell itself, together with the actual body of the mollusk, is hidden deeply in the mud, seldom less than three inches, and generally eleven or twelve inches from the surface. In this position it would be unable to respire were it not for the elongated tube, which projects through the mud into the water, and just permits the extremities of the siphons to show themselves, surrounded by the little radiating tentacles which betray them to the experienced shell-hunter."