Another suggested that the stone was removed in invisible particles by the constant action of currents in the water, produced by vibratile cilia seated on the soft parts of the animal; but this supposition was found untenable on examination.
Actual observation in the aquarium has proved that the second hypothesis is the true one. M. Cailliaud in France and Mr. Robertson in England have demonstrated that the Pholas uses its shell as a rasp, wearing away the stone with the asperities with which the anterior parts of the valves are furnished. Between these gentlemen a somewhat hot contention was maintained for the honour of priority in this valuable discovery. M. Cailliaud himself used the valves of the dead shell, and imitating the natural conditions as well as he could, actually bored an imitative hole, by making them rotate. Mr. Robertson, at Brighton, exhibited to the public living Pholades in the act of boring in masses of chalk. He described it as “a living combination of three instruments, viz., a hydraulic apparatus, a rasp, and a syringe.” But the first and last of these powers can be considered only as accessory to the removing of the detritus out of the way, when once the hole was bored, the rasp being the real power. If you examine these living shells, you will see that the fore part, where the foot protrudes, is set with stony points arranged in transverse and longitudinal rows, the former being the result of elevated ridges radiating from the hinge, the latter that of the edges of successive growths of the shell. These points have the most accurate resemblance to those set on a steel rasp in a blacksmith’s shop. It is interesting to know that the shell is preserved from being itself prematurely worn away by the fact, that it is composed of arragonite, a substance much harder than those in which the Pholas burrows. Yet we see by the specimens before us that such a destructive action does in time take place, for some of these have the rasping points much more worn than others, many of the older ones being nearly smooth.
The animal turns in its burrow from side to side when at work, adhering to the interior by the foot, and therefore only partially rotating to and fro. The substance is abraded in the form of fine powder, which is periodically ejected from the mouth of the hole by the contraction of the branchial siphon; a good deal of the more impalpable portions being deposited by the current as it proceeds, and lodging as a soft mud between the valves and the stone. Mr. Hudson,[28] who watched some Pholades at work in a tide-pool in the chalk, observed the periodic ejection of the cloud of chalk-powder, and noticed the heaps of the same material deposited around the mouth of each burrow. The discharges were made with no regularity as to time. Mrs. Merrifield[29] records a curious fact. “A lady, watching the operations of some Pholades which were at work in a basin of sea-water, perceived that two of them were boring at such an angle that their tunnels would meet. Curious to ascertain what they would do in this case, she continued her observations, and found that the larger and stronger Pholas bored straight through the weaker one, as if it had been merely a piece of chalk rock.”
Mr. Ross, of Rhyl, having a Pholas in his aquarium, prepared a piece of wood, by excavating a shallow cavity, about a quarter of an inch deep, in which he set the animal, whose shell was two inches long. “After a short time the animal attached its foot to the bottom of the hole and commenced swaying itself from side to side, until the hole was of sufficient depth to allow it to proceed in the following manner:—It inflated itself with water, apparently to its fullest extent, raising its shell upwards from the hole; then holding by its muscular foot, it drew its shell gradually down. This would have produced a perpendicular and very inefficient action, but for a wise provision of nature. The edges of the valves are not joined close together, but are connected by a membrane (extension of the mantle), and instead of being joined at the hinge (umbo) like ordinary bivalves, they possess an extra plate, attached to each valve of the shell, which is necessary for the following operation:—In boring, this mollusk, having dilated itself with water, draws down its shell within the hole, gradually closing the lower anterior edges until they almost touch. It then raises its shell upwards, gradually opening the lower anterior edges, and closing the upper, thus boring both upwards and downwards. The spines are placed in rows, like the teeth of a saw; those towards the lower part of the shell being sharp and pointed, while those above, being now useless, are not renewed.”[30]
RED-NOSES.
In this limestone cliff we shall find other borers, for you may see even at a considerable distance how holed and honeycombed its surface is; the cavities being so numerous, so close, and so irregular in their direction, that the whole face of the rock is fashioned into small sharp-edged shapeless points. Nor need we be long in finding the industrious masons who thus rough-point acres upon acres, nay miles upon miles, of limestone rock. Here in ten thousand orifices you discern little double-tipped knobs of crimson flesh, which, as soon as you disturb them, shoot at you a column of water and then disappear within their fortress, having exhausted their artillery. The fishermen know them well, and use them for bait, applying to them the familiar but expressive soubriquet of Red-noses.[31]
It is not so easy to get at these as at the Pholades, because of the superior hardness of the stone which they excavate. With the chisel, however, we need not fail of uncovering a few, especially as their burrows are but shallow. Here they are, half-a-dozen in a block as big as your fist. Ugly, uncouth, bemired, the valves not nearly containing the shapeless flesh, they are not particularly attractive creatures, maugre the brilliant hue of their blushing siphons. Like Mrs. Merrifield’s Pholas, these Saxicavæ habitually break into one another’s houses, as we see here, and even cut one another’s shells and bodies through and through most ruthlessly. They will live very well out of the rock, and may be kept a considerable time in the aquarium.
There is no doubt that the burrowing Mollusca are slowly but surely effecting changes in the configuration of rocky coasts, by destroying the rock. It is true their excavations extend but a few inches in depth; but then, as the force of the elements readily breaks down the thin partitions left standing, and discovers a new face, so the borers are continually renewing their attacks on this; and so in time the cliffs are worn away, while the debris of impalpable mud is deposited upon the shallows, entering into new combinations, and filling up estuaries and harbours.