Along the margins of these shallow rock-pools with sandy bottoms, we see many little grey shrimp-like creatures, with thick-set arching bodies, which swim rapidly, and with much vibration of the abdominal feet, from weed to weed. If we catch one and lay it on the rock or on the wet sand, it appears very helpless, for its vertically thin shape and arched attitude preclude the possibility of its crawling: it falls upon its side, and vainly struggles round and round as on a pivot, or makes aimless jerks by throwing out the tail. It is of a greenish blue, or glaucous colour, marked with red dots. This is the common Locust Screw,[75] which is so confined to sea-water, that it has been affirmed that if put into fresh water it presently dies. But the experiments of Mr. Robertson negative this statement:[76] he has found that five out of seven, after being eighteen hours in rain-water, continued to live upon being returned to salt water.
I delight to trace the manifestations of parental love in these tiny forms of animal life. It had long been known that the females of this species display a solicitude for their infant offspring, which they carry for some time about with them; but the following very pleasing details of maternal manners have recently been observed by my relative, Dr. James Salter, and by him communicated to the learned zoologists who are now engaged upon the history of this order:—
“On catching a female with live larvæ, nothing is seen of the progeny till the parent has become at home in the aquarium, when the little creatures leave her, and swim about in her immediate neighbourhood. The plan I have adopted to watch this curious habit of maternal protection has been to place a single individual in a bottle of sea-water. After a time, and that soon, the little crustacean seems at ease, and swims slowly about, when the young fry leave, and swarm around her in a perfect cloud; they never leave for more than half or three-quarters of an inch, and as she slowly moves about they accompany her. If, now, one taps the sides of the bottle with one’s finger-nail, the swarm of larvæ rush under their parent, and in a second are out of sight. The parent now becomes excited, and swims about quickly, as if trying to escape; but on letting the bottle containing her rest quite still on a table, she soon gets composed, when out come the young larvæ again, and swim about as before. This may be repeated as often as the observer wishes, and always with the same result. I have only seen this in one species, but it is quite a common species in Poole Harbour, and I have watched the interesting habit many times.”[77]
Every little stream and ditch of running water in our lanes and fields abounds with a species, which, except for the fresh-water habit, and a little difference in the shade of colour,—the latter being of a yellowish, instead of a bluish grey,—is so absolutely identical with the Locust Screw, that, notwithstanding it is registered under a name of its own, Gammarus pulex, it is difficult to believe that the two are not of common parentage. Certain facts observed in Sweden give colour to such a suspicion. The large inland seas of fresh water, Lakes Wetter and Wener, in the south of that country, are situated on high ground, and have the surface of their water 300 feet above the level of the Baltic, whereas the bottom is 120 feet below such level. In these lakes (which appear to have been lifted up with the gradual uprising of the country) have been found several genera and species of Crustacea, three of which are AMPHIPODA, which are affirmed to be identical with marine ones, viz., Gammaracanthus loricatus (Sabine, Ross, Kröyer), Pontoporeia affinis (Lindström), and Gammarus cancelloides (Gerstfeldt).
The first is now only known to exist in the Arctic seas, the second in the Baltic, and the last was found in Lake Baikal, in Central Asia. It is therefore suggested by Lovèn, that the land was raised so as to convert these waters from marine bays into inland lakes. These marine species were retained within the basins, the waters of which have since been changed, through the agency of springs, into fresh water; and with the gradual transfer of the water, the habits of the animals have also changed gradually, and that without any outward alteration of form. Professor Lovèn thinks that there is sufficient evidence to show that this change in the condition of these lakes must have taken place during the great glacier period, at a time when the animals now found in it (and which are known at this day only to inhabit the extreme north) could have lived in the same latitude as the south of Sweden. The evidence of these fresh-water lakes suggests that similar changes in the relative position of sea and land may have been the cause of our having fresh-water Crustacea nearly allied to marine species in our rivers and inland streams.[78]
Crawling about perpendicular faces of rocks, generally at a considerable distance above high-water mark, we may find a crustacean, not very remotely allied to these, of rather large size.[79] It is broad and flat, a form which marks it as belonging to another order, though in many peculiarities of structure agreeing with its near neighbours the high-backed Screws. The present species crawls readily with its fourteen short strongly-hooked feet, with which it clings to every little roughness of the stone. It swims much less effectively: indeed, I have never seen it spontaneously in the water; and those which I have captured on the rocks, and transferred to an aquarium, have died after a very brief interval. They have little agility under any circumstances, and trust rather to craft than to power for their safety. When alarmed, they instantly gather in their feet, and drop from their hold; and, as I have said that the positions in which they are found are for the most part perpendicular surfaces, such as cliffs and sea-walls, their one trick is often successful.
Thus the Ligia makes a decided approach to an aërial or terrestrial mode of life; which, indeed, is strictly the habit of some species with which every cultivator of flowers is only too familiar. I refer to the little Woodlice, or Buttons, or Sows, as they are variously called, so abundant in gardens, especially in pits and frames, and so annoying for their depredations on our cherished plants. These garden pests are scarcely to be distinguished as to their structure from the semi-marine Ligia, particularly those flatter and softer species (Porcellio), which do not roll their body into a complete ball.
In all these creatures we find maternal care. The females carry their eggs in a sort of pouch or sac, placed beneath the breast, which opens when these are hatched to afford exit for the infant progeny. These have from the first the same form and appearance as the adult, except that they have at first six pairs of feet instead of seven. They cling about the body of the mother, and are carried by her in her rovings with affecting solicitude.
About the sloping ledges of low-lying rock, especially those which are covered with young mussels, so closely that between their blue-black shells, packed and tied together with byssus-threads, you can scarcely thrust a penknife, we very commonly see slender Worms of a vivid grass-green hue gliding in and out. Their movements are very graceful, and if we examine them closely, we discern the presence of a multitude of minute points along each side of the body, which in turn are thrust out and as quickly retracted. If we put the little creature into a vase of water, we see fresh powers and organs of locomotion; for the back bears a double series of leaf-shaped fins, overlying one another, which act as broadoars rowing the lithe and graceful worm through the sea.
This species[80] is not more than three inches in length; but there is another[81] much less common, which is larger, richer in colours, and altogether a finer species. A specimen which I lately found was about a foot in length when crawling, and nearly a quarter of an inch in width. The whole body was divided into distinct segments, which in this individual were about two hundred and ten in number; though, according to MM. Audouin and Milne Edwards,[82] they sometimes amount to nearly five hundred, and the worm has been found two feet in length. The head is small, terminated by two pairs of very minute antennæ, but surrounded at what we may call the neck by four pairs of rather long feelers (tentacular cirri), with a fifth pair which are minute and rudimentary.
The colours are very beautiful. The leaf-like fins of the sides, which are somewhat heart-shaped, are of a yellowish green hue, occasionally clouded with blackish; the middle portion of the back, which is exposed, is of a rich brown, but flushed with the most glowing iridescence of blue and purple; while the whole under parts are of a pearly flesh-colour. As it crawls over the stones, it throws its body into the most elegant lateral curves, while its suppleness and great length cause it to cling close to the rock; and thus its outline takes the form of every projection and depression over which it is wandering.
When disturbed, and often without any apparent provocation, we see the under side of the tiny head rise from the ground, swell out, and turn itself inside out, as you turn a stocking, until a great pear-shaped bag is protruded, fully eight times as long, and thrice as broad as the entire head. Its whole surface is rough and papillose, and around its extremity, which is the largest part, there is a row of small knobs or warts. This curious organ is found under similar conditions in very many Annelids; it is commonly called the proboscis, or evertile œsophagus; but in truth it is a special and peculiar apparatus, with little analogy with anything found in higher animals: it is in some species furnished with strong horny teeth, and is doubtless employed for the capture of living prey, and the conveyance of it to the stomach.
The tail of the specimen I refer to was evidently being renewed after having been accidentally lost. For the body-segments ended abruptly, and were followed by a portion, not more than an eighth of an inch long, white, and excessively delicate; but which, when examined with a powerful magnifier, displayed a division into segments, each segment carrying its proper cirri; I could count twenty segments within that minute space, the last of which carried the usual pair of stylets.
I find among my notes a record of a specimen of Phyllodoce, which, if not identical with this, was closely allied to it, in which I observed the continuance of vitality after the severance of the creature into parts. It was sent to me from Torquay, when I lived near London. When it arrived I found that about an inch and a quarter of the anterior extremity was detached from the remainder, which measured about four inches. The former was motionless, contracted, and seemed lifeless; the latter moved freely. I put both into an old aquarium. The long posterior portion glided about among the stones for two days, exactly like a living healthy animal; the anterior part remained motionless and contracted until the third day, when I saw it also gliding over the stones in a most lively manner, rearing its head, and feeling about in the manner of a caterpillar. Eight days after its arrival, the head portion was still active and apparently healthy, but the hinder part had become motionless and was evidently dead. I find no further record of the case, and probably the anterior part ultimately died without reproduction; but the length of the period of its survival in apparent vigour, renders it not improbable that in the open sea, under the influence of abundant oxygen, and suitable food, the wanting parts might have been renewed to the fore-part, if not to both.
To return to my more recent captive, however. I killed it for cabinet preservation by putting it into fresh water, where it presently died, with the noticeable circumstance that it threw out mucus in such profusion that the whole body was enveloped in a mass, much thicker than itself, of clear jelly, excessively tough and tenacious.
The most common members of the class Annelida, that we meet with in these situations, are different species of the genus Nereis, which are for the most part worms of considerable size, usually brown or green, with a changeable metallic lustre above, and brilliantly pearly beneath. They have a distinct head, of a squarish form, terminating in two swollen fleshy knob-like antennæ, and furnished with four pairs of thread-like tentacular cirri, which project on each side like a cat’s whiskers. The body is plump, though somewhat flattened, and bears on each side a row of fleshy foot-warts, which are pierced for the extrusion of the curious bristles that are so characteristic of these marine worms.
We can scarcely turn one of these flat stones which lie half-buried in sandy mud at the water’s edge without finding one or more of this tribe. Let us try. Here at once is a specimen, one of the finest as well as one of the commonest of all.[83] The upper surface is of a warm fawn-brown; but the beautiful flashes of iridescent blue that play on it in the changing light, and the exquisite pearly opalescence of the delicate pink beneath, are so conspicuous as to have secured it the title of “Pearly,” par éminence. As you gaze upon it you see the great dorsal blood-vessel or heart, as a dark red line running along the middle of the back. This, at irregular intervals of five to fifteen seconds, contracts almost to invisibility, and then expands again, taking the form of a string of beads in each process.
This, however, is only a part of the great circulating system in this fine worm. In it the red blood flows from the tail towards the head; at the under side of the body, there is a similar vessel (the ventral), through which the blood flows from the head towards the tail. Besides these, there are four other longitudinal vessels in the interior of the body, one above and one below the intestine, and one on each side, all of which are connected by transverse branches. Thus there is a double circulation. “The great dorsal,” says Dr. Williams, “the reservoir of the centripetal streams of the body, may be likened to a right ventricle (the lungs cut off), and the great ventral to a left ventricle. The duty of the former is to collect the refluent blood of the system; of the latter to circulate it again.”
Our Nereis is furnished with two pairs of dark blue eyes, and with an evertile proboscis, which consists of two segments. The one first protruded is beset on its upper side with some largish horny points, arranged in two groups, which are continued round to the under side in a band of irregular lines of points. The second segment has four groups of points, the groups of oblong outline, radiating from the strong black jaws, which are protruded last, and can be widely opened and closed.
Here, too, we have an Annelid of very different form and appearance.[84] It is a mean-looking worm, about an inch and a half long, of flattened shape, blunt at each end, apparently covered with a smooth skin of a dull brown colour; but on being touched it presently throws itself into elegant serpentine curves, and then what appears to be the upper skin is seen to be composed of a great number of round flat membranous plates or shields, arranged in two rows overlapping each other. These, though of large size, are attached to the body only by a small point in the centre of their under side, so that when the animal moves, the edges of these shields are lifted, and reveal their true structure, sliding upon one another in a singular manner.
In captivity I have found the animal inert, prone to seek concealment in the corners of the glass vessel beneath other animals or stones, and remaining still for a long time together; but, if disturbed, manifesting impatience and much agility, swimming through the water with strong lateral undulation, as already described. I have been struck with the deciduous character of the broad leaf-like shields. As soon as the animal was in my possession, one and another of these appendages was cast off at almost every time that I roused it to action; and even when allowed to remain quiet, the denuding process went on, so that in a few days it was nearly half naked. This process of self-mutilation will sometimes go on till scarcely a single shield remains out of the double series.
This dull-coloured Scale-worm presents peculiarities of structure which are eminently worthy of careful examination. The body, like that of the other species we have just been considering, is composed of many rings or segments, each of which bears on its upper surface a little wart or tubercle on each side, to the summit of which is slightly attached the circular leaf-like shield just described. Beneath the shadow of this broad shield the segment projects laterally into another tubercle, whence issue the organs of motion, which in these Annelids are of highly curious structure. In this species they consist of a bundle of straight bristles of exquisite tenuity, and having a silky lustre; they are of unequal length, the upper ones being the longest, and the pencil diminishing pretty regularly downwards. The microscope shows them to be about one hundred in number in each bundle; and each bristle to be a delicate oar consisting of a slender stem, apparently tubular, dilated at the tip into a narrow blade terminating in a minute hook. A fibrous structure is seen to run diagonally across the blade, the edge of which is set with minute barbs pointing outwards, resembling very fine and very short bristles. The whole bundle is connected with a long slender rod of cartilage, which, doubtless acted upon by proper muscles, moves to and fro through the muscular sheath, protruding the bundle of bristles, or entirely retracting them within the body. The bristles, when protruded, slightly diverge, so that the dilated blades of these hundred oars strike with full force upon the water in rowing, if such indeed is the nature of their action, as some have supposed. In the act of withdrawal, however, into the narrow compass of the muscular sheath, the blades slide one upon another, so as to present a surface considerably diminished. But why is each bristle hooked and barbed? The obvious supposition is that these organs act like the pole of a ferry-boat in the shallows; the barbs, pointing outwards, serving to catch any roughness of the surface, and thus to push the animal by their resistance, while the terminal hook may serve to pull in an analogous manner. But then the barbed and hooked edge is the upper one of each oar, a circumstance which renders such an explanation at first sight unsatisfactory. Perhaps, however, the habit of the animal of living under stones, may in some measure account for it; the edge and point of each bristle being intended to act upon the surface of the object above it, beneath which it is forcing its way.
A secondary pencil of bristles also issues from the upper side of each of these tubercular sheaths, so as to project diagonally upwards and outwards beneath the edge of the shield-leaf. This pencil consists of between thirty and forty bristles, which are connected with a cartilaginous rod exactly similar to that of the primary bundle. These bristles run off to a very fine point, and each one is marked on both sides with serratures so delicate as to be only just discernible with a magnifying power of 300 diameters. There are thus about 270 movable bristles of exquisite structure in each segment, which multiplied by 36, the number of segments in the body, gives a total of 9720 distinct organs of motion possessed by this animal, besides the broad leaf-like shields, which perhaps may act as fins, and the short tentacular feet, one of which is placed beneath the primary bristle-sheath, on each side of the segment.
By dissection after death, I have found the proboscis in the form of a long oval sac, very muscular and firm, of a blackish hue, lying in the anterior part of the body, which, on being cut open, showed four horny teeth placed in its neck so as to meet each other in the centre. I have never seen the protrusion of this organ during life, but if it is capable of being wholly everted, as in many species, these teeth must be turned on the outside, where they would seem to be of little use. Possibly, however, the version may be only just sufficient to bring these teeth to the margin, when their action might be made very efficient by the slightest alteration in the contraction of the sac.
Some species of this genus construct for themselves enveloping tubular cases, out of mucus given off from their bodies, and fragments of shells. In these they ordinarily live concealed, but are sufficiently active when they leave their houses. Some shine with a phosphorescent radiance in the dark.
In the great class of Worms, special organs for locomotion are often altogether lacking; and, where found, are only rudimentary. Alternate swellings and contractions of certain portions of the body are a great source of progression; and these are effected by the fluids of the interior, which are driven to a given part of the cavity, and momentarily imprisoned there by the contraction of the circular tegumentary muscles before and behind it; the body swelling at that place. “The muscles of the integument are then excited to action, and the fluid is forcibly compressed forwards or backwards, according to the direction of the muscular agency. Nearly all Annelids are struck with paralysis when this fluid is made to escape from its cavity by a puncture through its external walls. The power of voluntary motion is suspended; the body of the worm becomes passive and flaccid.”[85]
These details convey but a feeble and imperfect notion of the numerous and elaborate contrivances which are so profusely bestowed upon these mean and grovelling worms; but they show how careful is the Creator of their well-being; how lavish of His mercies towards His meanest creatures. How unreasonable, then, is it for those to doubt His ever-watchful and unerring tenderness towards themselves, who have been made the objects of His redeeming love, and who, on the ground of redemption, “are of more value” in His sight, than all these His lower creatures put together!
A mile or two eastward of Babbicombe and Petit Tor, in from ten to fifteen fathoms water, there lies a stretch of flat stony bottom, reaching away from the island known as the Ore Stone, towards the mouth of the river Exe. This is a bit of ground to which a boatman whom I occasionally employ often resorts with the dredge, and rarely or never without a fair harvest of curious and interesting creatures. Among other things he brings me from time to time numerous specimens of what Yarrell calls the Bimaculated Sucker;[86] the propriety of which name will be evident the moment you examine one of these little fishes alive. Cuvier named the group in which they are found (a group comprising very few species) Discoboles, because the ventral pair of fins are united so as to form a circular disk or saucer, by means of which the fishes have the power of adhering firmly to any solid object larger than the circumference of the disk. Yarrell substitutes the term Cyclopteridæ for the family group; the name Cyclopterus, used for the principal genus, expressing the same thing, that the fins are united in a circular fashion. The word bimaculatus, signifying two-spotted, alludes to two remarkable and conspicuous oval spots of dark purplish brown or red, situated one on each side of the body, just behind the pectoral fin. These spots, being comparatively large, and separated from the general colour of the body by a well-defined white ring, constitute a very striking and beautiful feature in these little fishes.
Their form is flattened; they have a broad shovel-shaped muzzle, prominent eyes, looking rather upwards, and the head much widened behind; the head, indeed, constitutes more than one-fourth of the entire length; and at its hinder part, or where the gills open, is far wider than any other part of the fish. They rarely exceed an inch and a half in length. The general hue above is pale red; but in some specimens they become a nearly uniform lake-purple, in others the hue is a clear orange; while yet in others it is almost white. A band of white, bounded by darker lines, almost invariably connects the two eyes. Frequently the hue of the body is varied by clouds and patches of dark reddish brown, which patches have a tendency to assume a constant pattern, quite recognisable when you look at a good many specimens together. In the hinder half and on the sides the ground colour is apt to be minutely divided or mottled, the interspaces being of a delicate azure or lilac; and when examined with a lens, the whole surface seems sown with gold dust. The dorsal fin is set very far back, and, as well as the caudal, is prettily pencilled with dark brown: the under parts are pearly white.
The eyes are exquisitely beautiful, and as they are prominent, very movable, and especially as they are moved quite independently of each other, they at once attract and fix the admiration of the beholder. The large pupil is of a deep lustrous green, the iris of the most brilliant orange gold, and the whole set as it were in the midst of a globe of the purest glass. On the whole I scarcely know of a more attractive little fish than this.
This little Sucker is easily reconciled to captivity. I have in my aquariums some individuals which were captured nearly a year and a half ago; and they are still in the full enjoyment of health and activity. They are pleasing little things: they scuttle from spot to spot with a spasmodic sort of bustle, wagging their tails much from time to time, so that in movement as well as in form they remind one of tadpoles. They are generally seen, however, anchored to the sides of the vessel, or to the prominent knobs of the stones by their sucker, and here they remain for an indefinite time, sometimes for hours, at others only a few seconds, throwing their beautiful eyes about in insatiable curiosity, fanning the water with their transparent pectorals, or whisking the painted tail about. Now and then they make a sudden snap, doubtless at some passing animalcule, with an audible sound, and the emergence of a bubble of air from the top of the water, whence the muzzle is frequently projected. They have an awkward habit of throwing themselves out of a shallow vessel; and if you are not on your guard you may find your little pets dead and dry on the carpet. They are inquisitive little things; if a new stone or shell or tuft of serpulæ is put into their vessel they soon discover it, and may be seen exploring it in every part; and it is amusing, when you are examining them with a lens, to note how thoroughly mutual is the investigation; for you can see by the direction and motion of the eyes that the little fish is watching you as interestedly as you are observing him.
I do not think that the adhesion of the ventral sucker is effected exactly as has been supposed, by a vacuum produced in the area of the united ventral fins; but by the combined action of some minute fleshy sucking disks, which are arranged in two groups, in front of and behind the united fins. The conjoined fins do not appear to me to make a vacuum. The fish has a curious habit of coming to the surface of the water, and there floating perfectly still, back downward, the entire belly-surface dry. The ventral disk is then seen as a shallow cup, quite dry and shining. If touched, the little fish hurries along the surface, with some splashing, till it acquires impetus enough to go under at an oblique angle, when it presently turns over, and adheres to the bottom, or side of some stone. I have seen this practice frequently, but only, I think, at night.[87]
This principle of a vacuum produced by the retraction of the centre of a fleshy disk, while the margins remain in close contact with a solid body, is of extensive application in the lower forms of animal life, and especially in the class Echinodermata, comprising what are popularly known as Star-fishes, Sea-urchins, and Sea-cucumbers.
I have on other occasions noticed the elaborate and wonderful mechanism of the sucker-feet as they appear in the commoner species of the class. I need not therefore repeat those details, but look at a few other particulars in the economy of the animals whose locomotion is dependent on this curious contrivance.
There is a pretty little species abundant enough hereabout, chiefly affecting shores on which numerous angular masses of stone lie irregularly scattered and heaped one on another. Yet they seem to have a predilection, for they do not occur in all our localities even though these conditions be not lacking. Livermead Point, and the south side of Anstey’s Cove, beneath the cliffs, are favourite spots for them, the former especially, where we can find the little Gibbous Starlet,[88] for such is its name, at all times of the year, when the tide is sufficiently out. The retiring tide here leaves a shallow pool of considerable area, which then continues to run out by a narrow channel among the rock boulders, a winding rivulet of salt water; along whose borders, by turning over the loose blocks, scores of this pretty Star are exposed, clinging to the wet sides and roofs of the dark passages by means of their sucker-feet. Forbes has given two figures of the species, but manifestly taken from dead specimens, and from very small ones too. He says, “large specimens measure only an inch across;” from which I infer that on the shores of the Isle of Man, where he was familiar with it, the Starlet does not attain the dimensions it reaches on our mild southern coast. He indeed alludes to one in Mr. Ball’s collection, which measured one inch and five lines in diameter. Specimens, however, of this size are quite common with us, nor would one of an inch and a half be looked upon as at all exceeding the modest and proper range of the species.
It is of a pentagonal figure, with the margin a little receding between the angles, but not so as to cause the latter to form distinct arms. The body is flat below, and plump and cushion-like above, of a yellowish olive hue, with the very edge of a golden orange tint, while a spot of the latter colour, a little out of the centre, marks the situation of a remarkable organ called the madreporiform tubercle, the proper use of which has not as yet been satisfactorily determined. Just at the extremity of each angle, but a little below, is situated a wart of rich crimson hue, which is supposed to be an eye, being seated on a small ganglionic swelling of a nerve that passes along the ray. It is true no crystalline lens has as yet been detected on the pigment dot, either in this species or any other (for the specks are found in the same position in all the proper Star-fishes), but they are manifestly of the same character as similar specks in Rotifera, and other humble animal forms, which in some examples are connected with an indubitable lens. It might seem at first as if the situation of these eyes were not very favourable to vision; but, in truth, they command the ground just before and around the ray-tip, and also the water in a horizontal direction; and as there is one at each of the five angles, the entire circumferent space is viewed at each moment. Add to this, that the animal is in the habit of very frequently turning up the tip of one or other of its rays, when the range of vision would take in the zenith; and we shall perceive that no position in any other part of the body could be so suitable for the location of eyes as these selected. It is not to be supposed that distinct definition of objects is attained by these rudimentary organs; but the animal is probably conscious of the difference between light and darkness, and may also discern the sudden approach of any object, either by its interception of light, or by its colours, though its features and form may be indistinct. Such a degree of visual perception, though very imperfect in our esteem, may be of great use to this sluggish creature, and amply sufficient for its need.
The whole upper surface of this Starlet is covered with six-sided convex plates, each of which is crowned by a group of short blunt spines; the number in each group varying from one to nine; four to six, however, most commonly; arranged in a diverging tuft. Over the madreporiform tubercle, the tufts are stouter, and bend towards each other, as if to protect this delicate organ, which, whatever its function, is grooved with sinuous furrows, visible only with a considerable magnifying power, like the rounded masses of coral from the tropical seas, called brain-stones.
On the under surface similar stout short spines are arranged in transverse bands across each ray, interrupted, however, by the central furrow, which is perforated with two ranges of orifices to give issue to the sucker-feet, which thus form a double row. These organs do not differ importantly either in structure, function, or appearance, from those in the Cross-fishes, in the Sea-cucumbers, or in the Urchins.
In health and activity the whole upper surface is covered with a forest of short pellucid tubes, closed at the tips, which protrude from pores in the plates, and stand erect, moving, however, to and fro at the will of the animal; they are evidently filled with fluid. I cannot find any notice of these organs: they are probably connected with respiration.
What a wonderful piece of mechanism is a Sea-Urchin! Accustomed as I am to the multitudinous contrivances and compensations that present themselves at every turn to the philosophic naturalist, often as surprising and unexpected as they are beautifully effective, I am yet struck with admiration at the structure of an Echinus whenever I examine it anew. A globular hollow box has to be made, of some three inches in diameter, the walls of which shall be scarcely thicker than a wafer, formed of unyielding limestone, yet fitted to hold the soft tender parts of an animal, which quite fill the concavity at all ages. But in infancy the animal (and, of course, its box, as this must be full) is not so big as a pea; and it has to grow till it attain its adult dimensions. The box is never to be cast off, and replaced by a new one; the same box must hold the infant and the veteran Urchin. The limestone, not being a living tissue, but an inert earth, can grow only by being deposited. Now the vascular tissues are within, and the particles they deposit must be on the interior walls. This would indeed augment the amount of limestone in the box, but it would be at the expense of the contained space. The thicker the walls, if thickened from within, the less room in the cavity; but what is wanted is more room, ever more, and more. The growing animal feels its tissues swelling day by day, by the assimilation of food, and its cry is, “Give me space! a larger house, or else I die.”
How is this problem solved? Ah! there is no difficulty. The inexhaustible wisdom of Jehovah the Creator has invented a beautiful contrivance for the emergency. The box is not made in one piece, nor in ten, nor in a hundred; six hundred distinct pieces go to make up the hollow case; all so accurately fitted together, that the perfect symmetry of the outline is not broken; and yet, thin as their substance is, they retain their relative position with unchanging exactness, and the slight brittle box possesses all requisite strength and firmness.
Each of these symmetrical pieces of shell is enveloped by a layer of living flesh, a vascular tissue of exceeding thinness, which passes up between the joints where one meets another, on every side, and not only so, but actually spreads itself over the whole exterior surface. So that when you take up an Urchin into your hand, and having rubbed a small space clear of spines, look on it; you have not, as you may suppose you have, exposed the surface of the shelly box, but only the flesh that covers it; yet this is so transparent and colourless, so inconceivably thin, so absolutely adherent at every point, that its presence will not be discernible to feeling or sight, without the aid of high microscopic powers.
This being so, the glands of the investing fleshy tissue secrete lime from the sea-water, which holds it in solution, and constantly deposits it, after a determinate and orderly pattern, on every part of the surface of each shelly piece; the inner face, the outer face, and each of the sides and angles of the polyhedron grow together, and all so evenly, that while the dimensions increase, both of thickness and superficies, the form characteristic of that individual piece is maintained with immutable mathematical precision. Thus the volume and capacity of the box grow with the growth of the individual segments, and it ever keeps the globose shape at first imposed upon it.
But this is but a small part of the mechanism of this interesting tribe. If you put into a basin of sea-water one of the pretty kind[89] which we find so abundantly under stones at low water,—whose green spines are tipped with rosy purple, like the tentacles of an Anthea,—you will presently observe it marching majestically along by means of the hundreds of sucker-feet, which it possesses in common with the Star-fish. Now, if you have in your cabinet the empty box of an Urchin of this same kind, and taking it in your hand, hold it up to the light, and look into the cavity from the under or mouth side, you will have a very interesting spectacle. The light streams in through a multitude of minute holes, as smooth and regular as if drilled with a fairy’s wimble; and these holes are arranged in a pattern of elegant symmetry. They run in lines, like meridians, from pole to pole of the flattened globe; but instead of being set at uniform intervals, they constitute five principal sets or bands, with blank intervals between, about twice as wide as the drilled bands. Then each band comprises two series, each of which contains a double row of orifices. These last, again, do not constitute a single unbroken line, but an interrupted or zig-zag line, which is, in fact, made up of a number of short diagonal rows; three holes in each diagonal, set one after another.
Put the living and the dead together. These tiny orifices, as minute as the point of the finest cambric-needle could make in a bit of paper, afford exit to the suckers, which are, of course, equally numerous. Through these pass the slender pellucid tubes, filled with elastic fluid, which carry at their tips a flat ring of calcareous shell, affording to each the form and firmness to make each one an adhesive sucking disk, in the centre of which a tiny vacuum is created at will by muscular retraction.
But this is not all. Again, look at the living Sea-Urchin. It bristles with the rosy-tipped spines, which have a satiny lustre, owing to the reflection of the light from the delicate ridges and furrows with which the whole is fluted, like an Ionic column in miniature. How they are all moving, and swaying to and fro on their bases, quite independently of each other, however, making circles and traverses in the water with their points, as the mast-heads of a ship do among the clouds in a gentle swell, when seen from the deck. Professor Agassiz fell into the egregious blunder of supposing that the spines were the organs of locomotion in the Urchins, denying, with much contempt, the theory which attributed this office to the suckers. One can only wonder whether he ever saw a living Urchin in motion, as one moment’s glance at the phenomenon is sufficient to prove how utterly his theory was false, as Forbes has well shown.
However, let us once more observe the empty denuded box from the cabinet; and now on the exterior. Between the principal bands of pores (called ambulacra, from their fancied resemblance to walks in a garden), as also between the two series which constitute each band, the space is studded with hemispherical warts, of very diverse size, which look as if turned in ivory, unpolished; and each wart is crowned with a smaller wart of like material, but bearing the most perfect polish.
At the bottom of each spine there is a cavity exactly fitting this second wart, and equally highly-polished in its interior. During life the spine was seated on the wart, not united to it, but moving upon it in all directions, with perfect smoothness and freedom; a ball and socket joint, in fact. It was held in its place by an investment of muscle, which completely enclosed both the wart and the base of the spine, having one insertion in the unpolished wart, and the other in a remarkable ring or shoulder of the spine, visible just above the socket.
These are but two or three salient points of interest in the structure of this little unvalued, disregarded creature. I could relate much more; indeed, I think it would not be difficult to write a bulky volume of the history and biography of a single Sea-Urchin, of which every page would display the glory of God. But I have not space for that here.
We sometimes, but very rarely, find on this coast a very lovely form of this class of animals, the Rosy Feather-star.[90] It consists of ten long attenuated arms, radiating from a common centre, composed each of about forty slender joints of stone, and each joint carrying a pair of diverging beards, also many-jointed, all of which together, by their number and arrangement, give to the arm the aspect of a beautiful feather. Around the central point of radiation, a small cup-like body gives origin to the arms, which are double, a pair springing from a single basal joint. Within the cup the soft parts of the animal are chiefly located, the organs of the vital functions; and from the convex surface spring a number of jointed stony threads, like necklaces, much shorter and slenderer than the arms, which serve as claspers, gripping and holding firmly the projections of the rock, by means of strong curved claws with which they are terminated, in shape like those of an eagle.
The whole elegant creature is of a lively rose tint, interrupted by patches of bright yellow, disposed with no regularity or apparent order; the whole, both the yellow and the rosy portions, studded with crimson dots. Edward Forbes, if I rightly understand him, considers these dots to be ovaries, which he estimates at upwards of 57,000 in number.