Part 8

P. H. GOSSE, del. LEIGHTON, BROS.

LIMPET. PURPLE. SLIT-LIMPET. ]

My readers are, I dare say, familiar with the pretty myth which professes to embody the discovery of the purple dye. The Tyrian Hercules was one day walking with his sweetheart along the shore, followed by her lap-dog, when the playful animal seized a shell that had just been washed up on the beach. Its lips were presently dyed with a gorgeous purple tint, which was traceable to a juice that was pressed out of the shell-fish. The lady was charmed with the colour, and longed to have a dress of it; and, as wishes under such circumstances are laws, the enamoured hero set himself to gratify her, and soon succeeded in extracting and applying the dye, which afterwards became so famous. I have elsewhere recorded my own experiments on the stain yielded by the Purpura before us, with the remarkable changes through which it passes before the sunlight fully brings out the colour. The use of cochineal makes us independent of molluscan dyes, and the matter is merely one of antiquarian interest, or a question of zoological chemistry.

EGGS OF DOG-WINKLE.

Perhaps you may be more interested in the development of the Dog-winkle. Under the ledges of rocks we find in abundance groups of little yellow bodies, resembling ninepins in shape, set on their ends in close contact with each other, and varying in numbers from three or four to a hundred or upwards in a group. Some of them are tinged with purple at the tips; and while sometimes you find them closed, and full of a yellow creamy substance, at others they are open at the top, and empty.

These are the egg-capsules of this mollusk, and some very unusual circumstances connected with the birth of the progeny, and their development within these cases, have been discovered by Dr. Carpenter. Each capsule contains 500 or 600 globules that cannot be distinguished from each other at first; but only twelve to thirty of these are developed into young animals, though their united bulk ultimately equals that of the whole mass. The greater number of these globules are not real eggs, but only “yolk-spherules,” destined to afford nutriment to the true embryos, which greedily swallow them, after certain changes have taken place, and increase rapidly in bulk. It is curious, however, that they do not advance in development during this absorption of nutriment, but are, so to speak, arrested until a great augmentation of size is thus attained; then they quickly acquire the form of little free-swimming nautiloids, closely like those of the Doris and Eolis, a form which indeed is common to the early stages of all the known higher mollusca, however various may be their adult conditions.

LIMPET.

Here are the familiar Limpets, too: let us look at them awhile. They are not generally very attractive in appearance, the shell being coarse and rubbed, especially in the larger specimens; and in an aquarium they do not live long, and are so inert as to afford no amusement even while they survive. Yet we occasionally find examples prettily coloured; and there are facts in their economy which make them worthy of a few moments’ notice.

If you look carefully over the rocks, especially when these are of a somewhat soft nature, as the slates and shales, you will find oval depressions, sometimes but just discernible, at other times sunk to the depth of an eighth of an inch, corresponding in outline to the shell of a Limpet; and in many instances you will actually see a Limpet imbedded in such a pit, which it accurately fills. Strange as it may seem, it has been ascertained that these cavities are formed by the animals, which make them their ordinary resting-places, wandering away from them nightly to feed, and returning to them to rest early in the morning.

The force with which a Limpet adheres to the rock is very great, especially when it has had warning of assault, and has had time to put out its muscular strength. Réaumur found that a weight of twenty-eight or thirty pounds was required to overcome this adhesive force. His experiments seem to prove, however, that its power is mainly owing, not to muscular energy, nor to the production of a vacuum in the manner of a sucker. If an adhering Limpet were cut quite through perpendicularly, shell and animal, the two parts maintained their hold with unabated force, although of course a vacuum, if there had been one, would have been destroyed by the incision. The power is said to reside in a very strong glue, a very viscid secretion, deposited at the will of the animal. “If, having detached a Patella,” says Dr. Johnston, “the finger be applied to the foot of the animal, or to the spot on which it rested, the finger will be held there by a very sensible resistance, although no glue is perceptible. And it is remarkable that if the spot be now moistened with a little water, or if the base of the animal be cut, and the water contained in it allowed to flow over the spot, no further adhesion will occur on the application of the finger: the glue has been dissolved. It is nature’s solvent, by which the animal loosens its own connexion with the rock. When the storm rages, or when an enemy is abroad, it glues itself firmly to its rest; but when the danger has passed, to free itself from this forced constraint, a little water is pressed from the foot, the cement is weakened, and it is at liberty to raise itself and be at large. The fluid of cementation, as well as the watery solvent, is secreted in an infinity of miliary glands with which the foot is, as it were, shagreened; and as the Limpet cannot supply the secretion as fast as this can be exhausted, you may destroy the animal’s capacity of fixation by detaching it forcibly two or three times in succession.”

THE GILLS.

If we remove one of these Limpets from his selected area of rock,--which we may readily do, notwithstanding the strength of his cement, if we take him at unawares, and give him a smart sudden horizontal rap with a piece of wood, or a moderated blow with a hammer,--we shall obtain a view of a structure well worth looking at. The animal is essentially like a Trochus or a Purple inhabiting a conical shell; only in this case the cone is low and simple, whereas in the others it is tall and slender, and rolled into a spire. One of the most curious peculiarities in the Limpet is its gill or breathing organ. This, we perceive, completely encircles the animal, forming a ring interrupted only at one point. It lies in the fold between the mantle and the foot, commencing on the left side of the neck, and passing quite round the body, parallel with the edge of the shell, in front of the head, till it terminates close to the point where it began. It is a long cord closely beset with tiny leaflets, and thus forming a continual plume. Each leaflet, conical in outline, is permeated with blood-vessels, and clothed with minute cilia, whose constant vibrations cause the circumambient water ever to play over the surface of these organs in ceaseless currents, bringing fresh supplies of oxygen to be respired; and this is absorbed by the blood through the thin membrane by which they are protected.

There is a very pretty little shell, not uncommon in deep water off these coasts, but rarely found by the shore collector, though it does occasionally venture to peep at daylight at the verge of extreme low-tide. It is the Slit Limpet, which by the older naturalists was placed in close alliance with the Limpets proper, as if a member of the same family. They were, however, deceived by paying too exclusive attention to the form of the shell, which is a cone, somewhat rounded, and nearly simple, the summit being slightly turned over in a backward direction. The margin of the shell is delicately notched, the points being the extremities of the radiating ridges; for the entire surface is covered with reticulations, one series of alternate furrows and ridges proceeding from the summit to the margin, and another series crossing these at right angles, running round the shell parallel with the margin. The animal has its sides ornamented with short fleshy processes, and possesses two symmetrical gill-plumes, one on each side. It is rather attractive in appearance, but I cannot tell you anything of its manners; for though I have kept specimens in the aquarium, they are so habitually sluggish, and so reluctant to allow one a peep beneath the edge of the jealous shell, that I could learn nothing about their ways;--if indeed they have any.

PHOLAS HOLES.

Another curious form closely related to this is the Keyhole Limpet, whose shell is of a long oval outline, of a lower cone, reticulated, like the Slit Limpet, but pierced at the summit with a double hole, or rather a perforation apparently made of two holes broken into one, something like a keyhole. This orifice, like the slit in the former case, is for the discharge of the effete water taken in in breathing.

See! here is the soft red sandstone lying in great beds, pierced through and through with smooth round holes, just as if bored with a carpenter’s auger, big enough to admit a man’s thumb. What agency has been in operation to effect these perforations? Let us try to discover.

A few good blows with the stout hammer on the chisel-head serve to split off a great slice of the coarse red sandstone. The holes run through its substance, but they are all empty, or filled only with the black fœtid mud which the sea has deposited in their cavities. Yes; these are too superficial; they are all deserted; the stone lies too high above low-water mark: we must seek a lower level. Try here; where the lowest spring-tide only just leaves the rock bare. Ha! now we have uncovered the operators. Here lie, snugly ensconced within the tubular perforations, great mollusca, with ample ivory-like shells, which yet cannot half contain the whiter flesh of their ampler bodies, and the long stout yellow siphons that project from one extremity, reaching far up the hole towards the surface of the rock.

We lift one from its cavity, all helpless and unresisting, yet manifesting its indignation at the untimely disturbance by successive spasmodic contractions of these rough yellow siphons, each accompanied with a forcible jet d’eau, a polite squirt of sea-water into our face; while, at each contraction in length, the base swells out, till the compressed valves of the sharp shell threaten to pierce through its substance.

PLATE 6.

P. H. GOSSE, del. LEIGHTON, BROS.

RED-NOSE. FINGER-PHOLAS. ]

HYPOTHESES OF THEIR FORMATION.

Strange as it seems, these animals have bored these holes in the stone; and they are capable of boring in far harder rock than this; even in compact limestone. The actual mode in which this operation is performed long puzzled philosophers. Some maintained that the animal secreted an acid which had the power of dissolving not only various kinds of stone, but also wood, amber, wax, and other substances, in which the excavations are occasionally made. But it was hard to imagine a solvent of substances so various, and to know how the animals’ own shells were preserved from its action; while, confessedly, no such acid had ever been detected by the most careful tests.

Others maintained that the rough points which stud the shell enable it to serve as a rasp, which the animal, by rotating on its axis, uses to wear away the stone or other material; but it was difficult to understand how it was that the shell itself was not worn away in the abrasion.

Another zoologist, rejecting this hypothesis, maintained that the edges of the mantle and the short thick foot are the instruments employed; and that, though these fleshy organs seem little fitted for such work, they are really endowed with the requisite power in the shape of crystals of flint which are deposited thickly in their substance. Strange to say, however, other accurate observers fail to detect these siliceous crystals, and therefore reject the hypothesis.

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, 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 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.”