CHAPTER XVIII.

ECHINODERMATA.

Primeval Sea-stars.- Feather-stars.- Snake-stars.-Star-fishes.-Their Suckers and Mode of Locomotion.-Their Skeleton.-Their Victims and their Enemies.Sea-Urchins. Structure of their Shell.-Their Dental Apparatus.-Pedicellariæ, or Sea-Cucumbers.-Metamorphoses of the Echinodermata.

Ar that far-distant period of the earth's history when the swampy lowlands were covered with those thickets of calamites and stigmarias whose remains have given birth to the coal strata of the present day, the bottom of the ocean was paved in many places with crinoid star-fishes, whose bodies, branching out into delicately feathered bifurcated arms, were affixed like flowers to a slender articulated stalk.

Their petrified skeletons, imbedded in countless numbers in many of the calcareous strata of our island, bear witness to their ancient importance; but the beautiful and antique race of these Lily Encrinites and Pentacrinites is now reduced to but one single representative in the British seas-the rosy feather-star, whose long and delicately fringed ray, and deep rose colour dotted with brown, may serve to give us an idea of the beauty of the submarine landscapes at the time when the bottom of the sea was peopled with gigantic specimens of the same class. Attached in its infancy to a stalk like its mightier predecessors, it swims freely about at a later period, by alternately contracting and extending its closely-feathered arms. It is found all round our coasts, and is frequently brought up in from ten to twenty fathoms water, attached to different kinds of seaweed, which it lays hold of by means of the claws which tip the filaments that clothe its body.

Thus the crinoid star-fishes have mostly disappeared, but the asteroidea, forming the two great subdivisions of the snake

stars and star-fishes, that have usurped their place, abound in the modern ocean, and frequently, like their predecessors of old, cover the bottom of the sea with a living carpet.

The snake-stars are essentially distinguished from the true star-fishes by the long serpent or worm-like arms which are appended to their round, depressed, urchin-like bodies. They have no true suckers with which to walk, their progression being effected (and with great facility) by the twisting or wriggling of their arms, which are, moreover, in many species furnished with spines on the sides, assisting locomotion over a flat surface. These arms are very different from those of the true star-fishes, as they are not lobes of the body as in the latter, but mere processes attached or superadded to the body. In the sand-stars and brittle-stars they are simple; but in the Scotch Argus or Warted Euryale, of which the adjoining woodcut represents a segment, each of the five rays is branched again and again, so that the whole resembles a bunch of

Sand-star.

serpents' tails. While swimming, this strange creature spreads and stretches out all its branches to their full length, but when a desirable prey comes within their reach, it suddenly contracts their Gorgon coils and entraps its victim as in the meshes of a net. As the expanded Argus measures about a foot in diameter, it may well be supposed that it is no contemptible enemy for the small fishes it may meet with on its way.

The brittleness of the snake-stars is highly remarkable, for at the slightest touch they separate themselves into pieces with wonderful quickness and ease. Touch the common brittle-star,

Warted Euryale.

aud it flings away an arm; hold it, and in a moment not a process remains attached to the body.

The common brittle

star,' says Edward Forbes, often congregates in great numbers on the edges of scallop banks, and I have seen a large dredge come up completely filled with them; a most curious sight, for when the dredge was emptied, these little creatures, writhing with the strangest contortions, crept about in all directions, often flinging their arms in broken pieces around them; and their snake-like and threatening attitudes were by no means relished by the boatmen, who anxiously asked permission to shovel them overboard, superstitiously remarking that "the things wern't altogether right." There can be no doubt that, thanks to this facility of dismemberment, the brittle-stars disappoint many a hungry foe of at least part of his meal, and wriggle out of his reach while he feasts on one of their cast-off arms.

Inferior view of Asterias Rubens,

The locomotive appara

tus of the true star-fishes is of a much more complicated structure than that of the snake-star; for as they are born not to swim, but slowly to creep along A upon the bottom of the sea,

the grooved under-surface of their arms is provided with two or four rows of small worm-like suckers, which, alternately extending and contracting, serve to drag the body after them. The strength and activity of these little organs afford an entertaining spectacle when a starfish is placed on its back in a plate filled with seawater. At first the creature remains motionless;

At A part of the feet is removed. a mouth, e spines. for, offended by the rough

a

treatment it has undergone, the feet have all shrunk into the body, leaving nothing visible but series of minute tubercles, but soon they are seen to emerge like so many little worms

ORGANISM OF THE STAR-FISH.

135

from their hole, and to grope backwards and forwards through the water, evidently seeking the nearest ground to lay hold of. Those that reach it first, immediately affix their suckers, and, by contracting, draw a portion of their body after them, so as to enable others to attach themselves, until, pulley being added to pulley, their united power is sufficient to restore the star-fish to its natural position.

This act of volition is surely remarkable enough in an animal which hardly possesses the rudiments of a nervous system, but the simple mechanism by which the suckers are put into motion is still more wonderful. Each of these little organs is tubular, and connected with a globular vesicle filled with an aqueous fluid, and contained within the body of the star-fish, immediately beneath the hole from which the sucker issues. When the animal wishes to protrude its feet, each vesicle forcibly contracts, and propelling the fluid into the corresponding sucker, causes its extension; and when it desires to withdraw them, a contraction of the suckers draws back the fluid into the expanding vesicles. All these little bladder-like cavities are connected

Section of a ray of Asterias Rubens,

Showing the arrangement of the calcareous pieces. a oblong calcareous plates united in the median line. b smaller lateral plates.

with vessels, which communicate with a vascular circle surrounding the mouth; while the internal walls, both of the suckers and the system of communicating vessels, are furnished with vibratory cilia, through whose agency a continual circulation of the fluid they contain goes on within them, and serves to aërate the blood.

Not only the suckers, but also the rays from which they proceed, are extremely flexible in every direction, for the skeleton of a star-fish, or that part which remains when all the soft flesh has been removed, is a wonderfully beautiful structure, consist

ing of hundreds of nicely-fitted calcareous pieces arranged in a regular pattern, perfectly symmetrical in all its parts; so that the supple animal finds no difficulty in making its way through the crevices of a rocky shore, or in traversing the intricate tangles in search of prey. The march of the sea-star is indeed but slow, and hosts of little marine animals on which he would willingly feed, no doubt escape his voracity, as they have been gifted with a greater agility; yet his table is richly furnished, for there are hosts of molluscs that are not only more tardy than himself, but even firmly rooted to the ground, and have nothing to oppose to his attacks but the passive resistance of their closed valves.

This defence, however, is frequently of no avail, for star-fishes are not unfrequently found feeding on shell-fish, enfolding their prey within their arms, and seeming to suck it out of its shell with their mouths, pouting out the lobes of the stomach, which they are able to project in the manner of a proboscis. Possibly the stomach secretes an acrid and poisonous fluid, which, by paralysing the shell-fish, opens the way to its soft and fleshy parts. Thus the star-fish is a sworn enemy to oyster banks, and consequently also an indirect enemy to man; but fortunately he himself is an object of pursuit to greedy crabs, cephalopods, and fishes, who, however, are frequently at some pains to catch him; for it is no easy matter to detach him from the rock to which he clings, and the spines which frequently cover his coriaceous back are likewise no despicable means of passive defence.

The sea-star might be called a flattened sea-urchin with radiated lobe, and the sea-urchin a contracted or condensed seastar, so near is their relationship. Still there are notable differences between them. Thus, in the sea-urchins, the digestive organs form a tube with two openings, while in the sea-stars they have but one single orifice. The mouth of the latter, which may be so dilated as to admit large mollusca in their entire shell, has only toothed processes projecting into its cavity, but that of the sea-urchin is a masterpiece of mechanical contrivance. Fancy five triangular bones or jaws, each provided with a long projecting moveable tooth. A complicated muscular system sets the whole machinery going, and enables the jaws to play up and down, and across; so that a more effective mill for grinding down the food cannot well be conceived.