At low-water, turning over stones and looking into rock-crevices, we are sure to come across members of the Echinodermata—the creatures with tough and rough or spiny coverings, popularly known as Star-fish and Sea-urchins. There are many forms of these to be found on the British coasts, though some of them are peculiar to deep water, and not likely to fall in our way, unless it be their dead bodies washed up to our part of the shore. But we can obtain a fair knowledge of the class to which they belong, from the specimens we can find living their lives in our own littoral zone. Here, hunched up into an almost globular form under this drooping mass of leathery wrack, is the common Five-fingers, Cross-fish, or Star-fish (Uraster rubens). Turning him on his back we see the reason for the contracted condition of his five rays: in the hollow thus formed he holds no less than three specimens of the Purple or Dog Winkle. Why? He is a glutton, and is eating those three poor mollusks at one sitting.
Not many years ago we all believed literally the tales that were told of the Star-fish swallowing oysters as large or larger than itself. It was well known that they caused havoc to oyster and mussel beds, and that seemed the most likely way in which the valuable bivalve would be destroyed. Some went so far as to assert that Five-fingers waited his opportunity to catch the oyster gaping, and then slipped in one of his fingers, and so prevented the shell closing. It was left to the imagination to picture that same finger hooking out the native and swallowing it in the approved fashion—off the shell.
The Uraster’s mouth is small, and the integuments tough and not capable of great distention; but its stomach is a most accommodating organ, though a very delicate one, and when the Star has come upon food too large to pass through the mouth to the stomach, the stomach passes through the mouth to the food. It surrounds the victim with its fine membrane, pours out its gastric juice, and having reduced it to a fluid condition, re-absorbs the whole, and returns to its natural position inside the Star. That is a wonderful process, but it is quite a common one, and you will certainly catch the animal in the act before you have long been shore-hunting.
This is probably the way in which the securely boxed up oyster falls a victim to Five-fingers. The oyster’s powerful adductor muscles keep the valves closed, and appear to defy any burglariously-disposed creature of its own size; but Five-fingers’ gastric juice is a penetrating solvent which paralyzes the muscles and kills the oyster. The elastic hinge then opens the shell automatically, and allows Five-fingers to make an unresisted entrance, and a short end of the oyster.
As we have shaken off the dog-winkles, the Star-fish takes in his stomach for safety, and we are enabled to have a look at his exterior. When we say that he has five rays proceeding from a common centre, we have said well-nigh all that is to be said about his form. But the minutiæ of the organs disposed over those rays, and within them—for their interiors form part of the general body-cavity—requires much describing and explaining. The creature has no legs, yet he moves with considerable celerity in any one direction as easily as another, and inequalities of surface present no difficulties to him. And yet the five rays, from their stiffness, are practically useless for this purpose; but on the under surface of these rays are hundreds of pliable and active little suckers, worked by hydraulic power, and it is all one to them whether they have to walk on rock, weed, or glass, up or down, across the floor, or under the ceiling.
Looking at the underside of this Star, we find that each of the rays is deeply channelled along its centre. Only the true Stars have got this channel; the Sand-stars and Brittle-stars have not, neither have they got the wonderful suckers; but along each side of the channel, under each of Five-fingers’ arms, there are two rows of soft filaments that bend and wave in any direction, and that end each in a little knob containing a tiny limy plate. By means of this little plate each knob is converted into a sucker, similar to those by which trades-people suspend their goods from the surface of their plate-glass shop-fronts, but worked by water instead of air. There are hundreds of these to each ray, and all act in unison, so that real progress is made when Five-fingers’ olfactory sense informs the sucker-feet of the direction in which food may be sought.
Ah, you say, has it a nose? No, it has not; but experiments have shown that the entire underside is sensitive to odours. At the tip of each ray there is a spot that is ordinarily spoken of as its eye, but it has no true eye, though these spots are sensitive to light. Its mouth is in the centre of its under surface, and opens directly into the stomach, which has branches running into each of the rays. The vent for the undigested particles of food and for waste, is on the upper surface.
Near the junction of two of the rays on the upper surface will be seen a round stony knob, which is sometimes taken for the creature’s eye. This is not a very wild shot at its purpose, though it is entirely a wrong one, for as placed it certainly does suggest some such function. Its real office could not suggest itself to any person unacquainted with the internal economy of the Star-fish. Looked at through a lens, it will be found to have a number of minute pores in its surface. Strange as it may seem that the Star-fish should require such a convenience, this is really a filter. Scientific men honour it with the important-sounding name of the “madreporiform plate,” because its tubes resemble somewhat those of the madrepore coral. I have already referred to the hydraulic system by which the sucker-feet are distended and worked, and this is the “intake” of the supply, as a water-company would call it. Within these is a tube, running near to the creature’s mouth on its lower surface, and connecting with a ring of tube that surrounds the mouth, and sends out a branch to each of the five rays. To this branch-pipe all the sucker-feet in a particular ray are connected, and the pressure can be so regulated as to alternately distend the sucker-feet, or to leave them partly empty and flaccid.
Upon one occasion, when I was describing these arrangements of the Stars to a jocular friend, he said the idea of having a big mouth that let in water freely, and a number of minute mouths that let it in slowly, reminded him of the poet Cowper’s whim in making a large aperture for his big hares to pass through, and a small one for the little hares. He thought the mouth would have served both purposes; but as I pointed out to him, the water that the Star takes in involuntarily with its food goes into the stomach, where the food is retained and the water strained off by the mouth again. This water would contain grains of sand, vegetable débris, and other impurities, which would clog the delicate tubes and spoil a beautiful piece of mechanism. The water that percolates through the minute pores of the stony plate must be pure, and free from all extraneous matter, so that the special supply-pipe is a necessity. The scientific appellation of the sucker-feet is pedicels or ambulacral feet.
We must not omit to mention organs of another sort that occur in plenty among the sucker-feet, and for many years presented a puzzle to naturalists, who long regarded them as parasites—something foreign to the Star-fish. They are now understood to be pedicels that have been specialised to adapt them for particular functions. They consist of slender flexible stalks, ending in an enlarged head of three claws which normally converge to a point, but they are for ever opening and shutting and taking hold of something. They are analogous to those curious bird’s-head organs on some of the zoophytes, to which attention has already been directed. Their function is to take hold of seaweeds and other substances, until the suckers can be got to work; also to keep the sucker-feet clean by removing all matter tending to clog them and impede their efficient working.
The upper side of a common Star-fish is covered with rough or spiny plates, and bosses of carbonate of lime secreted by the creatures, and these take definite patterns in different species.
The Common Star-fish (Uraster rubens) is well-known for its rough orange-coloured exterior, and its profusion, in some seasons, upon certain parts of the coast. It swarms on oyster and mussel beds, and causes considerable annoyance to fishermen, who find it taking possession of the bait on their lines, and so keeping off the fishers’ rightful prey.
There is a less common but prettier species, the Spiny Star (Uraster glacialis), you may find among the rocks at low-water. It is much larger than the common Cross-fish, and in proportion the rays are longer, their sides more parallel, the upper side more distinctly spiny, and the colour a glaucous green, with variations towards violet. It is more angular-looking than the common species.
Another species is the Eyed Cribella (Cribella oculata), which has an upper surface quite free from spines or roughnesses, and of a purple colour.
These Stars go through a remarkable metamorphosis. In the year 1835, Sars, the celebrated naturalist, discovered a peculiar creature about an inch in length, to which he gave the name, Bipinnaria asterigera, and classed it among the Jelly-fishes. Nine years later, however, some further observations caused him to reconsider this view, and to regard the creature as more probably the larva of a Star-fish; and in the course of a few years this opinion was confirmed by the researches of Messrs. Koren and Danielssen.
The Sun Star (Solaster papposa) is really a glorious creature, with a broad central cushion of rich crimson, from which radiate from twelve to fifteen arms of the same colour, but with a band of lighter tint at their base. The upper surface is covered with a network of slightly raised lines, upon which are threaded, as it were, a great number of little cushions, supporting erect brushes of spines. It may be found at low-water, but is more frequently obtained from trammels set in deeper water, and from the fishermen’s lines. It is sometimes nearly a foot across from tip to tip of opposite rays.
In the bottom right-hand corner of the plate on page 93, will be seen a figure of the Gibbous Starlet (Asterina gibbosa), in which it will be seen that the figure of the Common Star has been considerably modified by the partial filling up of the angles between the rays, so that the body appears to be more extensive than the rays. This pretty species—it is represented natural size—is fairly plentiful in rock-pools where there is sand and a vigorous growth of coralline and fine weeds. In such pools it is not easily seen, owing to the manner in which it harmonises with its surroundings. It is covered with a short “pile” of spines, of a greenish-grey tint, with an indefinite shade of brown. It is cushion-shaped; and the underside is channelled from the five points to the central mouth. These channels are bordered with a row of spines on each side, to protect the double range of sucker-feet within.
In the same pools, among the rubbish at the bottom, under stones at low-water, and climbing about corallines and weeds, we shall be sure to find in plenty a little Brittle-star (Ophiocoma neglecta), of very attenuated proportions, and not exceeding an inch across, if you can get it to keep still whilst you measure it. It is exceedingly active, and all its tiny rays bend and wriggle at the same time.
The Brittle-stars pass to the other extreme from the Starlet, in modifying the five-rayed plan of the Common Star. Here the creature runs almost entirely into the five writhing arms, which leave but little material for the circular trunk, which looks, in truth, as though five active worms had simultaneously seized a minute button by its edge.
There are several other species of Brittle-star to be found between tide-marks, but they all share, more or less, the peculiarity which gives them the popular name. They are so “touchy” that you need scarcely do more than look at them to cause them to voluntarily snap off a part or whole of a ray, or several rays—and they commonly throw off the lot, if they commence self-mutilation. The amputated members are replaced by new growths, if the Star lives; for sometimes this act of renunciation of limbs that offend, is but a prelude to the extinction of vitality in the trunk.
In the illustration here given of the Granulate Brittle-star, it will be seen that the rays do not merge imperceptibly into the trunk, but are attached to it by a kind of dovetail joint on the upper side. Below, the arms, at their termination, form a ring, within which is the mouth, whilst the trunk acts as a roof above the mouth, and overhanging all round. The rays are composed of a series of joints, which allow free lateral action, or wriggling, but not much vertically. Each of these joints consists of four little plates, one each above and below, and one on each side. The side plates bear each from five to ten stiff and granular spines of varying length; and short tentacles come out beside the lower plates. These tentacles are not sucker-feet, like those of Five-fingers, but rigid, hooked processes; and there are no pedicellariæ with their snapping jaws. The mouth is a very extensive opening, but its area is largely occupied by the five jaws, the free ends of which extend upwards far into the body cavity, and are covered with rows of long, close-set teeth. These teeth, on the five jaws being brought together, must form a wonderfully efficient masticatory apparatus.
One of the commoner forms of these Brittle-stars is the Granulate Brittle-star (Ophiocoma granulata), represented in part in our illustration. I have seen crab-pots brought in with this species thickly coating the bottoms inside, and attached to well-nigh every bar; there must have been thousands in each “pot.”
An allied species that is more plentiful as an inhabitant of the littoral zone, is the Red Brittle-star (Ophiothrix rosula), which will be found sprawling over the under surfaces of big stones at low-water, in company with the Broad-claw Crab. Of this species Edward Forbes truly remarks:—
“Of all our native Brittle-stars, this is the most common and the most variable. It is also one of the handsomest, presenting every variety of variegation, and the most splendid displays of vivid hues, arranged in beautiful patterns. Not often do we find two specimens coloured alike. It varies also in the length of the ray-spines, the spinuousness of the disk, and the relative proportions of rays and disk; and in some places it grows to a much greater size than in others. It is the most brittle of all Brittle-stars, separating itself into pieces with wonderful quickness and ease. Touch it, and it flings away an arm; hold it, and in a moment not an arm remains attached to the body.”
Another species, the Long-armed Brittle-star (Ophiocoma brachiata), has the rays about twenty times the diameter of the disk, each consisting of three or four hundred joints; so that if one reckons up the four plates that go to make one joint, then adds to these the eight to ten spines on each joint, and multiplies the first total by say three hundred and fifty (the number of joints), and this second total by five (the number of rays), one gets a grand total of seventy thousand pieces, constituting merely the external covering of the rays of this small creature—leaving entirely out of the reckoning the internal bony framework upon which these are placed.
These Brittle-stars go through a peculiar stage of existence, prior to their assumption of rays. When summer is verging upon autumn, their minute larval forms may be gathered in a fine muslin net, from the surface of the sea. Gosse has given a description of this stage with admirable brevity. He says:—
“A painter’s long easel affords the only object with which to compare the little creature; for it consists of four long, slender, calcareous rods, arranged two in front and two behind, with connecting pieces going across in a peculiar manner, and meeting at the top in a slender head. On this shelly, fragile, and most delicate framework, as on a skeleton, are placed the soft parts of the animal, a clear gelatinous flesh, forming a sort of semi-oval tunic around it, from the summit to the middle; but thence downward the rods, individually, are merely encased in the flesh, without mutual connection. The interior of the body displays a large cavity, into which a sort of mouth ever and anon admits a gulp of water. Delicate cilia cover the whole integument, and are particularly large and strong on the flesh of the projecting rods.
“The appearance of this most singular animal is very beautiful; its colour pellucid-white, except the summit of the apical knob, and the extremities of the greater rods, which are of a lovely rose-colour. It swims in an upright position, with a calm and deliberate progression. The specimens which I have seen were not more than one-fortieth of an inch in length.
“From this form the Brittle-star is developed, but in a manner unparalleled in any other class of animals. The exterior figure is not gradually changed, but the star is constructed within a particular part of the body of the larva, ‘like a picture upon its canvas, or a piece of embroidery in its frame, and then takes up into itself the digestive organs of the larva.’ The plane of the future Star-fish is not even the plane of the larva, but one quite independent of, and oblique to it. Strange to tell, the young Star does not absorb into itself the body of the larva, which has acted as a nidus for it, but throws it off as so much useless lumber—flesh, rods, and all!”
Prof. A. Agassiz, however, would have taken exception to that last sentence, for he declared that “the whole larva and all its appendages are gradually drawn into the body, and appropriated.”
In the plate on page 93 there are two figures besides the Starlet—the Feather-star and a Sea-urchin. The Feather-star (Comatula rosacea) is really a deep-water form, but it has been taken occasionally within the littoral zone, and may occur there in the experience of the reader. It is undoubtedly the most beautiful of the entire group, so far as British waters are concerned, and it possesses a special interest for us, as being the only British representative of the Stone-lilies or Encrinites that so abounded in Palæozoic times that their remains make up whole strata, but of which, until the deep-sea explorations of recent years, no living European species was known. But the Feather-star, as shown in our illustration, had been long known, for in several localities round Britain and Ireland it came up abundantly in the dredge, yet no one suspected it was closely related to the Encrinites.
In the year 1823 Mr. J. Vaughan Thompson, when dredging in the Cove of Cork, brought up a tiny creature less than an inch in length, but which might have been one of these Encrinites, into which life and mobility had been infused. The discovery was hailed with joy by naturalists, and the little stranger was named Pentacrinus europæus. Thirteen years later Thompson came to the conclusion that his Pentacrinus was only the larval form of Comatula; and in 1840 Edward Forbes, Robert Ball, and C. Wyville Thomson were dredging in Dublin Bay, when the dredge brought up specimens of the so-called Pentacrinus in a more advanced stage than had been seen hitherto, and behold, some of these underwent the final change in their early history under their eyes: the Feather-star left its stalk and floated off, a true Comatula. Sir C. Wyville Thomson has given this interesting account of its progress from the egg condition:—
“The young escapes from the egg a pear-shaped free animalcule, swimming and gyrating rapidly through the water, large end foremost, by four transverse bands of cilia, and by a tail-like tuft of long cilia, which it uses somewhat in the style of a screw propeller. On one side of the body there is a large oval mouth, richly ciliated, and a short curved stomach. After swimming freely in this form for several days, a network of calcareous plates begins to appear, at length making a closed chamber in the wide end of the pear, and extending as a sort of stalk to the narrow end. The stalk now lengthens, and the creature loses its symmetrical form; it attaches itself to a stone or seaweed, and from the free, wide extremity, there springs a little circlet of branches—the arms of the second stage.”
On turning again to the illustration (page 93), it will be seen that the Rosy Feather-star, to give it the full title, is possessed of ten rays, or rather five rays each forking into two, and that these branches are pinnate, or feathered with little appendages which contain the ova. The ordinary organs are all contained in the central body, and do not extend into the rays as in Five-fingers.
The remaining figure in that plate is the Purple-tipped Sea-urchin (Echinus miliaris), which is a well-known inhabitant of rock-pools. It is enclosed in a stone box, which is a miracle of design, for although there is no elasticity about it, and it cannot be stretched, it yet serves the growing Urchin for years, and never cramps him. There is never any necessity for throwing it off, as the crabs and lobsters have to do repeatedly with their suits of armour. The nearest parallel to it in nature is the human skull, which although consisting only of a few pieces, enlarges in a similar manner to accommodate the growing brain.
It is remarkable how, in the whole sub-kingdom Echinodermata, all the wonderful variety displayed by the many species is found compatible with rigid loyalty to the dominating “number” principle: in these animals everything is governed by the number five. With a few exceptions, the rays are in fives, or multiples of that number; so are the jaws, the boundaries of the plates, and other details, as may be seen in any of the Stars to which we have alluded.
In the Sea-urchins we get an advance in that direction, for its stone box is built up of nearly six hundred five-sided[2] plates of lime, securely attached to each other by their edges, and fitting with such beautiful accuracy, that there is not the ghost of a crevice from base to crown of this wonderful cupola.
[2] Dr. Andrew Wilson, in his “Glimpses of Nature,” impresses upon his readers this “pentarchy” in the building of an Echinus, but curiously describes the plates as being six-sided. He evidently had none but living specimens to refer to when he thus wrote. It is only in a specimen from which the spines and skin have been carefully cleaned that the form of the plates can be seen.
But if there are no crevices there are many apertures—over five thousand of them in a full-grown Echinus esculentus. Forbes, many years ago, calculated there were 3,720 in a moderate-sized specimen; and his figures, though used in all the books since his day, do not appear to have been checked. But I have counted the pores in what I should describe as a moderate-sized individual, i.e., one that measured, when denuded of its spines, twelve inches in circumference, and find no less than 4,800. The calculation is as follows: ten bands, each consisting of eighty rows of six holes (10 × 80 = 800 × 6 = 4,800). The specimens that the crabbers take out of their crab-pots, and smash against the rocks, are commonly much larger than this. It always grieves me to see such wonderful structures destroyed in that fashion.
The five thousand pores are in pairs, each pair giving rise to one pedicel or sucker-foot, like those we described in the Five-fingers. The ten bands of pores are also arranged in pairs, the bands forming a pair being separated by about five rows of spines, and each pair of bands being separated from the next by twenty (4 × 5) rows of spines. These intervening spines are borne on two series of long, yet still five-sided, plates; the number of spines to each, in a growing specimen, varying; but from counting many of these I should suppose a fully-grown plate, from the middle of a series, would support twenty spines; at the top and bottom of the series, however, there are only two or three spines to each plate. These spines are not mere rigid outgrowths like the prickles on a chestnut bur; they are beautifully finished pieces of mechanism, with considerable latitude for movement in any direction. Although only about five-eighths of an inch in length, each is a beautiful column in alabaster, tapering slightly to the top, and decorated from near the base with a series of thirty (6 × 5) parallel rounded ridges. The bottom of this spine is hollowed out and polished perfectly, to enable it to move freely on the polished knob upon which it fits. These knobs are the bosses left on the shell when the spines have been cleaned off; the spines being held to them and moved by a circular band of muscular tissue.
If we look at the underside of the Urchin we shall find the mouth occupying the centre, with five polished white teeth protruding. Although these are not much to look at from outside, they form a large and complicated structure within, which goes by the name of the “Lantern of Aristotle,” because the famous Stagyrite appropriately compared its shape to a lantern. Within we find a set of organs similar to those described in connection with Five-fingers, much of the space being occupied with the water-vascular system by which the enormous number of sucker-feet are worked. The Urchin also possesses a great number of pedicellariæ which keep the upper parts of the huge sphere clean, by passing any particles of dirt from one to the other, until they are passed off altogether. The madreporiform plate is situate right at the summit of the edifice, near the five eyes and the vent. As its specific name suggests, this urchin is edible; it is boiled like an egg.
The Purple-tipped Urchin (Echinus miliaris) is depressed in form, and its outline would represent an oval from which one-fourth had been cut away, whilst E. esculentus would represent a circle from which about one-sixth had been abstracted. The skin of E. esculentus, when the spines are removed, is reddish; that of E. miliaris, a dusky greenish-grey. Miliaris is common in rock-pools and about the rocks at low-water; but esculentus is found in deeper water, though, from the frequency with which it is brought in by the crabbers for destruction, rather than throw it overboard where they find it, and from its empty house being rolled in by the waves, it is a fairly common object of the shore.
There is a rarer shore species, called the Purple Urchin (Strongylocentrus lividus), which excavates circular holes in the rocks large enough to house itself, spines and all. This is more plentiful in Ireland than on the English coasts; and it is remarkable not only for its excavating propensities, but also because it sheds its thick purple-spines annually, and produces a new crop.
Closely allied to the Sea-stars and Sea-urchins are the Sea-cucumbers, of which we have a number of native species, though many of them belong too exclusively to the deeper waters to be mentioned here. Several of the genus Cucumaria, however, may be met among the rocks, at low-water, on our southern coasts. One of these is represented in the accompanying plate, protruding from a crevice in the rock. It is the Sea-cucumber (Cucumaria pentactes), a species that requires a fair pair of eyes to detect it. Certainly, when seen for the first time, unless the finder had previously read about Sea-cucumbers, it would never strike him as being a relation of the Sea-stars and Urchins. There are no spines, no limy plates; instead, the body is soft and molluscous, so that it can progress by its alternate extension and contraction. But a careful scrutiny of the appendages encircling the mouth might awaken suspicion, for there are ten branching rays, and then it might be noted that the body has five distinct angles, and that these angles are pierced with pores not unlike those of the Urchins, through which protrude sucker-feet. This, he would consider, constituted a very strong case in favour of their relationship to the Echinoderms; and in this conclusion he would be in agreement with the scientific men, who have, however, also taken the Sea-cucumber’s internal arrangement into consideration. Another point which suggests affinity with the Sea-stars—especially with the Brittle section—is their trick when suffering from want of food or lack of oxygen in the water surrounding them, of throwing off portions of their body, and thus increasing their chances of life by their reduction of the area or bulk that has to be fed or refreshed. The animated Cucumber not only throws off its rays for such reasons, but also its mouth and dental apparatus, and its intestines and ovaries are turned out, and only an empty hollow bag remains. Should its prospects brighten through the access of food and the oxygenating of its surroundings, it will, in the course of a few months, reproduce these sacrificed organs, and make a fresh start with a new lease of life. This is a close connection of the tropical Beche de Mer, of which the Malays make Trepang, a very important item in their trade with China, by whom it is used as a choice article of food.
The creature to the left of the Sea-cucumber, on page 103, is the Dotted Siphon-worm (Sipunculus punctatissima), formerly included with the Sea-cucumbers, but now relegated by the systematist to the biological lumber-room, whose door is labelled “Vermes,” that limbo to which all sorts of creatures are sent who cannot be satisfactorily classified, in the hope that future discoveries may make their affinities more clear. The Siphon-worm has a cylindrical proboscis that is almost as long as its body, and a wreath of simple tentacles around the mouth.