ORDER CYCLOBRANCHIATA.
The breathing apparatus in this small group consists of a great number of little conical leaflets, arranged in a circle, more or less complete around the body, and attached beneath the margin of the mantle. The animal is covered by a shell varying much in dimensions and in structure in the different genera. They are all somewhat sluggish animals, adhering for many hours together to the surface of rocks, or other bodies, by means of the foot, which is large and muscular. All of the species inhabit the sea.
Family Patelladæ.
(Limpets.)
A conical shell is the distinctive character of this family; showing no trace of a spire, and destitute of any aperture or notch, by which other genera are known, which have shells of similar form and appearance. The shell, which is made out of one entire piece, quite covers the body; it is in the form of a widened cone, the apex of which is higher or lower, nearly central, or more approaching one end,—in different species. The animal is large in proportion to the shell, with a mantle, under the projecting edge of which, is a fringe of small leaves, that perform the office of respiration. The head is furnished with a large but short proboscis, and with two pointed tentacles, each of which carries an eye at the outer side of its base. The mouth is fleshy, and contains a long slender tongue, armed with spinous teeth, for the rasping down of the sea-weeds, of which the food of these animals consists. This organ in the common Limpet is described as a narrow ribbonlike-body, fully three inches long, of nearly equal breadth throughout, except at the tip, where it is soft and somewhat dilated. The width of this singular appendage is not more than one-twelfth of an inch; but the surface is armed through its whole length, with three parallel rows of spinous teeth, pointing backwards. The teeth of the middle row are cut into four points, but those of the external rows, which are not exactly opposite to, or continuous with, those of the middle series, but alternate with them, are cut into two points only.
TONGUE OF LIMPET.
(The upper figure represents a portion of the surface magnified).
"The first time," says Mr. Patterson, "we chanced to see this, we mistook it for some strange species of worm; but on examining several Limpets, the supposed worm was seen in all; and great was our astonishment when we discovered that we had, in every case, been looking at the tongue of the Limpet, and not at any intruder into the privacy of his conical fortress."[1]
This curiously toothed tongue "is never protruded beyond the margin of the lips. It seems to be used for rasping down the food; and in proportion as the anterior prickles are worn away in this operation, and absorbed, another portion of the tongue is brought forward to supply its place; but that there may be no deficiency in its length, we find the apex soft and vascular, where in fact a continual growth and addition are going on.
"When a phytophagous Gasteropod is about to eat, it thrusts forward, and to a certain extent, evolves, the spinous tongue, protruding at the same time the lip on each side, by which the tongue is compressed and forced into the form of the bowl of a spoon. The food is now taken hold on by the lips, drawn forwards, and retained by the prickly tongue, and simultaneously pressed against the upper horny jaw, by which means a portion is bitten off, sometimes with a very audible noise. The detached morsel is then passed along the tongue, torn and rasped down by its sharp prickles, and forced on by the peristaltic motion of the organ, and by the retropulsive action of the adjacent muscles, the mass is made to enter the gullet. At the entrance of this canal, there is an uvular caruncle, which is probably the seat of the animal's taste; and on its side a pair of lobulated salivary glands, or sometimes two pairs, which have each a single excretory duct to convey their peculiar secretion into its upper part, to lubricate and soften the mass. The gullet is a muscular canal, lined interiorly with a mucous coat, presenting, indeed, the same structure as the whole alimentary canal, and is generally plaited in a longitudinal direction."[2]
M. De Blainville considered that the organ of respiration in the Limpets was a vascular network, spread over the interior of a cavity, situated above the neck, with a wide opening in front. Hence he constituted the family into an order, which he named Cervicobranchiata, or neck-breathers. The opinion of so eminent a zoologist, adverse as it was to the received judgment of his fellow labourers in science, demanded a close investigation, which has been given by M. Deshayes and others. The result has been to show that such a cavity exists, with a structure similar to that of the Limpets, in many other Gasteropoda, which possess distinct and undoubted gills of the pectinate form, which I shall presently describe; and that there is no sufficient reason for believing that this chamber in the neck has any respiratory function at all: this office being fulfilled by the fringe of floating leaflets that encircle the body, as had been maintained by Cuvier and others.
In order to fit these little organs for the office which has been assigned to them, they are furnished with a multitude of cilia, microscopically minute, covering all parts of their free surface. By means of the constant undulating movements of these cilia, a perpetual current of sea-water is made to roll along each leaflet, communicating the requisite oxygen to the blood-vessels, of which it is mainly composed. The currents flow from the outer towards the inner edge, across the surface of each leaflet.
The doubts occasionally cast on what are received as established truths in science, though they may seem to unsettle our knowledge, and give a character of vagueness to it, must not be considered as inimical either to its progress or its solidity; not even when, as in the case just cited, they are found to be without foundation. They give rise to new and more careful examinations; to extensive comparisons of species with species, or of fact with fact; to satisfactory inductions of principles from observations; and often to the discovery of laws before unsuspected.
Genus Patella.
As the family consists but of this single genus, the characters already enumerated need not be repeated. The species are numerous, widely distributed over the globe, scarcely any sea being destitute of some, with the exception of the Arctic Regions, where none have been observed by voyagers. As usual, the largest species are found in the seas of the tropics. Deshayes in his Tables enumerates 104 living species, and ten fossil; several others have been added by subsequent naturalists, but as the genus is peculiarly liable to variation in the form, colour, and surface of the shell, it is very likely that many of these described species are merely varieties.
The animals of this genus have the power of wearing away, or of absorbing the surface of other shells, or of the rocks to which they adhere, and of thus forming sunken pits or depressions on them. The Patella cochlear of the Cape of Good Hope, is often found attached to a large species of the same genus, on the surface of which it forms a flat disk, exactly agreeing in size with the circumference of its own shell. To form these depressed disks, (of which there are so generally two on each larger Patella, one on each side of the apex, as almost to form a character of the species,) and to assist in the increase of its size, the animal appears also to absorb the coralline or other similar substances with which the larger shells are abundantly covered.[3]
COMMON LIMPET.
But we need not wander to the southern hemisphere for illustrations of this power. The most familiar shell-fish of our shores, the common Limpet (P. vulgata), will afford one equally good. Who has not seen the oval pits, sometimes but just discernible, at others sunk to the depth of an eighth of an inch or more, on the rocks of our coast, each accurately corresponding in shape and dimensions with a Limpet which inhabits it? I have wondered at them many times, not being then aware of the habits which have been ascribed to these animals, of wandering away from these pits, (which they have chosen for a home,) and of returning to them regularly again.
These habits were first made known by Mr. Lukis, a naturalist of Guernsey, to whom we are indebted for other interesting notes on the economy of animals. "The locomotion of the Limpet," he observes, "may be ascertained by marking one individual to avoid mistake, and then observe its cautious roaming and regular return to its favourite place of rest, where the shell will be found exactly to correspond with the surface of the rock to which it is attached. Here it will rest or sleep, and only relax its strong adhesion to the rock, when the muscular fibre becomes exhausted by long contraction, in which state a sudden blow, horizontally given, will easily displace it. A fact known to the fishermen and poor, who use them for food, is, that they are more easily collected in the night time than in the day. May not this be the period of roaming for food as well as when covered by the tide?
"The march of the limpet is slow and formal; and whenever the cupping process is renewed, the posterior end of the shell is brought in contact with the rock, which is of a soft nature, and will receive the impressions of its denticulations. The track of an individual placed under surveillance was thus made visible over a space of several yards, possessing the same regularity and disposition, and was further remarkable for the constant revolution on its left.
"The tracks of the limpet on granite and other hard rocks, present, at first sight, the same appearances; but, on a closer examination, they are found to differ. When first observed in 1829, a large portion of a fine-grained sienitic rock was traced over by these shells; the remainder was plain, and appeared varnished with a thin coating of some kind of fucus, without any markings upon its surface. As no patellæ were at first discovered, and the isolated situation of the rock prevented any from reaching it, I was at a loss to explain these appearances; but, after some search, a fissure was found at the north end, where five or six limpets had fixed themselves, each having a direct road leading to their pasturage-ground. By the help of a glass, the markings visible on the rock were discovered to be the remains of the above fucus, which had been eaten through or trodden down by these animals in their excursions, and which retained the indentures of their shells. The edge of the vegetable surface was then examined, and found to be nibbled in a circular manner resembling the anterior margin of the shells." [4]
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 to 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, are 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."[5]
This common limpet, though hard, coarse, and unsavoury, is largely eaten by the poorer classes on our rocky shores. It is easily procured in almost any quantity, between tides, and therefore is a good deal resorted to by those who have little or nothing better. The wretched inhabitants of the isles of Scotland, and of the Atlantic shores of Ireland, in particular, have often been preserved from actual famine by this miserable food. The quantity eaten as a regular part of diet is immense. Mr. Patterson, when residing, in July, 1837, near the town of Larne, in the county of Antrim, endeavoured to form some calculation of the quantity of limpets alone taken from the rocks about that part of the coast, and used as food. He had reason to believe that the weight of the boiled "fish" was above eleven tons! The weight, as carried from the beach, was, however, much greater, as there was to be added that of the shell, and the seawater which it contained. This, too, was exclusive of a probably equal quantity of periwinkles and whelks.[6]
Family Chitonidæ.
We find in this family a group of mollusca, which, possessing in their anatomical structure nothing very peculiar, present, in the covering by which they are protected, a form of shell quite anomalous, and such as to have given rise to a conjecture that in this family we have the link that connects the Mollusca with the Articulata.
The shell in the chitons consists of eight narrow transverse calcareous pieces, overlapping each other, and strongly implanted in a thick and fibrous border of the mantle, which surrounds the whole. The mantle itself is of a stiff leathery consistence, and though sometimes smooth, is more commonly covered with small scales, spines, or hairs. The elasticity of this investiture allows the animal to stretch and contract itself in crawling, and even to roll itself in a ball, in the manner of an oniscus or a hedgehog, the shelly pieces moving freely upon each other. To effect the various motions required, there are three muscles given off from each piece to the succeeding one, one in the line of the back, the others at each side. Thus the animal may be considered as enveloped in a coat of plate armour, and the name of coat-of-mail shell is sometimes applied to it by collectors.
SHELL-PLATES OF CHITON.
The chiton has no projecting head, nor any tentacles, but a kind of veil that surrounds the mouth; the eyes also are wanting. There is a very long ribbon-like tongue, armed like that of the limpet, with horny teeth. The gills consist of small triangular leaflets, set in a fringe, which runs along the furrow between the mantle and the foot. The foot is large, and extends the whole length of the animal.
In the family are comprised two genera, differing in the development of the shelly plates.
Genus Chiton.
The character which distinguishes the chitons proper from the Oscabrelles (Chitonellus), is that the dorsal plates of shell are comparatively large, much wider transversely than longitudinally, and all in contact with, and overlapping each other. This genus includes a large number of species, which are scattered over all seas, except in the very rocky shore, in greater or less abundance, fast adhering by the broad foot, exactly in the manner of limpets. The largest species are found on the tropical coasts of America, where some attain the gigantic dimensions of four, five, and even six inches in length. The Chiton spiniferus of Chili is said to reach the size last mentioned. The shells are much prized by conchologists, and they are consequently sought by collectors in foreign countries, though the operation is sometimes attended with danger. I have myself collected some kinds, of large size, on the shores of Jamaica, among sharp and rugged rocks, where the surf dashing in breaks over the naturalist at almost every wave, drenching him, of course, and often buffeting him against the rocks, and washing his prize from his hands the very moment he has detached it from its hold.
The mode in which Chitons are procured requires some skill and practice, as if they are touched without being detached in an instant, they increase their adhesion so greatly as to defy all efforts to remove them without lacerating the edges of the mouth, and thus spoiling them as cabinet-specimens. An old knife, that has the tip blunt and rounded, is the best adjunct. The operator must apply the point of this close to the extremity of the Chiton, without actually touching it; then, striking a smart blow with the palm of his other hand on the handle of the knife, the animal is dislodged by the shock, before it has any opportunity to confirm its hold. To prepare it now for the cabinet, it must be thrown into fresh water for several hours, and when quite dead, which may be known by the relaxation of muscular rigidity, the foot and all the soft parts must be cut out of the concavity of the mantle. The latter must then be placed on a narrow strip of board, exactly as if the animal were crawling, to which it must be tightly bound by threads passed round and round in every part, and laid to dry in the shade. Specimens prepared in this way will possess a natural appearance, and will never curl up in any state of the weather.
The flesh of the larger chitons is red and coarse; it is, however, eaten by the negroes of the West Indies, who compare it, by a certain exercise of imagination doubtless, to beef. There is a species found in the same locality, reported, I know not on what foundation, to be poisonous.
TUFTED CHITON.We have about fifteen species of this genus enumerated as British, of which one of the largest, as well as most common, or at least most generally distributed, is the Tufted Chiton (C. fascicularis). It is about three-fourths of an inch in length, with the shelly plates striated and granulated, and the margin of the mantle studded with little bundles of bristles, about nine or ten on each side. It varies in colour, but is generally greyish, or dusky olive. I have dredged it of large size in Weymouth Bay, attached to oyster-shells. A much smaller species is also common in that locality (C. cinereus), which occurs in the pools and on the ledges of the shore, near low-water mark.
When put into a glass of sea-water, the Chitons are sluggish, often remaining for days rolled up before they begin to adhere, and then remaining a long time inert on the same spot.