Part 4

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.[23] 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.[24] 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,[25] 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,[26] 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.[27]

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.

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,[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.

FOOTNOTES:

[16] _Triopa clavigera._

[17] _Doris tuberculata_, which the reader will see figured in Plate IV., in the centre of the foreground.

[18] _Eolis coronata_, figured in the upper part of Plate IV., above the Sea Lemon.

[19] Both these are depicted in Plate IV. That on the right of the picture is the spawn of _Doris tuberculata_; that on the left is the spawn of _Eolis coronata_.

[20] Alder and Hancock (DORIS).

[21] _Purpura lapillus_, sometimes known as the Dog-winkle. Three individuals, representing the varieties of colour, are seen in the middle of Plate V., and a cluster of their egg-capsules in the lower left corner.

[22] See _Devonshire Coast_, p. 60.

[23] _Trans. Micr. Soc._ (Ser. II.), vol. iii. p. 17.

[24] _Patella vulgata_, represented by two examples in Plate V., at the left side of the picture.

[25] _Emarginula reticulata_, of which a figure appears in the right-hand corner of Plate V.

[26] _Fissurella reticulata._

[27] _Pholas dactylus_; the principal figure in Plate VI., represented as exposed in its burrow by the splitting off of a portion of the limestone rock.

[28] _Zoologist_, p. 7819.

[29] _Nat. Hist. of Brighton_, p. 185.

[30] _Zoologist_, p. 6541.

[31] _Saxicava rugosa_; represented by the smaller figure in Plate VI.

III.

MARCH.

Perhaps the most effective aid to the investigation of natural history which the present age has produced is the invention of the aquarium, and particularly its application to marine forms of life. Depending on that grand principle of organic chemistry, of world-wide prevalence, that the emanations from animals and vegetables are respectively essential to the continued life each of the other, it was discovered that the relative proportions of number and bulk in which organic beings of the two kinds could healthfully live together was easily determined; and since the fact that the creatures were inhabitants of water, whether fresh or salt, presented no exception to the universality of the law, they had but to be placed together in a suitable ratio, enclosed in a vessel containing water, and an aquarium was established. Improvements in the form of the vessel, in the mode of exposing the contents to observation, in the impact of the rays of light, in the arrangement of the interior, and other points of value, have indeed been progressively made; whereby the practical availability of the invention for the purposes of experimental natural history has been augmented; but some of us have found little difficulty, even from the very first announcement of the discovery, in maintaining the collections of sea-water, with their living plants and animals, unchanged from year’s end to year’s end. I may be perhaps excused for observing, that I have at present in use a large tank, full of marine creatures, in which the water has been unchanged for four years, and on which I look with peculiar interest, because it was the first tank ever made for private use. This very aquarium has afforded, and still affords the opportunity for the observation of many interesting details of the structure and habits of the lower forms of animal life, details which constitute the basis not only of my works on marine natural history already published, but of the present series of papers also. We collect the creatures, indeed, abroad, and there gather up some broad facts of interest concerning their modes of life; but it is at home, in the quiet of the study, with conveniences and aids to examination, experiment, and record at command, that they must be studied. The aquarium becomes in fact an apparatus, whereby we bring a portion of the sea, with its rocks, and weeds, and creatures, to the side of our study-table, and maintain it there.

USE OF THE AQUARIUM.

Thus an opportunity of close and valuable familiarity with sea-productions is open to multitudes who have never seen the broad expanse of ocean, nor searched its prolific shores; and facilities for extending the bounds of zoological science are everywhere enjoyed, which till lately were restricted to a very few naturalists, whose residences were situated on certain favourable spots upon the coast. Yet both modes of investigation are necessary. He who has never seen marine animals except in the confinement of an aquarium, cannot but be conscious of many chasms in his knowledge, which are filled up by him who is in the habit of collecting his own specimens in their proper haunts; and who, by finding them _in ferâ naturâ_, can, when he studies them at leisure in his tanks, make such allowances as are necessary for the variations in habit which may be dependent on the difference between their present artificial, and their original natural, conditions of existence.

While we rejoice then in tanks and vases of crystal water, filled with the lovely forms and brilliant hues of sea-weeds and sea-anemones, I invite my readers to accompany me on a few hours’ visit to the charming creatures at their own homes. The season is propitious; the sun has just passed the vernal equinox, and the genial warmth of spring is diffusing new life into the cold blood of the animals that dwell beneath the waters; the equinoctial storms that lately raged have blown themselves out, and are succeeded by a quietude whose effect is delightfully seen in yonder mirror-like ocean: it is the time of spring-tide; and the near approach of the hour of lowest water will afford us unusual facilities for finding species only to be invaded under such conditions.