This class consists of five orders, all of which are marine. They are, with one exception, creeping animals, and the whole class is remarkable for having most of their members and general structure either in fives or multiples of five. Their skin is hardened by calcareous deposits, sometimes of beautiful microscopic structure: they have a digestive cavity, a vascular fluid system, and some distinct respiratory organs, so that they are comparatively of a high grade.

Echinodermata AsteroÏdea.

The AsteroÏdea, or Star-Fishes, which are the highest order, form two natural families, the StelleridÆ and OphiuridÆ, which comprise twenty-two genera.

The simplest form of the StelleridÆ is the common star-fish, with its flat regularly five-sided disk. A tough membrane, strengthened by reticulated calcareous matter, covers the back, and bends down along the sides, while the under-side of the body or disk, on which the animal creeps, is soft and leathery, with the mouth in its centre. In the other genera, although the body is still a flat, five, equal-sided disk, the angles are extended into long arms, broad whence they diverge from the disk, but decreasing rapidly in width to their extremities, so that the animal is exactly like a star with five long, equal, and flexible rays.

The backs of all the star-fishes are covered with most minute movable spines, and with microscopic organs like minute pincers, called pedicellariÆ, which are diffused generally over the surface, and form dense groups round the spines. They have a slender, contractile, calcareous stem, and a head formed of two blades, which they continually open and shut, the whole being coated with a soft external tissue. They grasp anything very firmly, and are supposed to be used to free the star-fish from parasites. In some species of Goniaster the pedicellariÆ resemble the vane of an arrow, and are so numerous as to give a villous appearance to the skin of the back.

On the under-side of each ray of a star-fish, a central groove or furrow extends throughout its whole length, and the semi-calcareous flexible membrane which covers the back and rays not only bends down round the sides of the rays, but borders both edges of the grooves. Upon these edges ridges of small calcareous plates beset with spines are placed transversely: they are larger near the mouth, and gradually decrease in size as they approach the point of the ray.

Interior to the spines, these ridges are pierced by alternate rows of minute holes for the long rows of feet, which diminish in size to the end of the ray. The feet are contractile muscular tubes communicating through the holes with internal muscular sacs, which are regarded as their bases. The sacs are full of a liquid, and when the animal compresses them the liquid is forced through the holes into the tubular feet, and stretches them out; and when the muscular walls of the hollow feet are contracted, the liquid is forced back again into the sacs, and the feet are drawn in. The liquid is furnished by a circle of small vascular tentacles, or sacs, surrounding the mouth, which are both locomotive and prehensile. From these a canal extends through the centre of each ray, which in its course sends off lateral branches to the bases of the feet to supply them with liquid. The whole of this system of vessels and feet are lined with vibratile cilia, which maintain a perpetual circulation in the liquid.

The toothless mouth on the under-side of the disk dilates so as to admit large mollusca with their shells. The short gullet and stomach are everted, protruded through the mouth, and applied round the object to be swallowed, which is then drawn in, digested, and the shell is discharged by the mouth. However, in three orders of this family there is a short intestine and vent. From the large stomach, which occupies the central part of the disk of the star-fish, a couple of tubes extend to the extremity of each ray, where they secrete a substance essential for digestion: the stomach is in fact a radiating organ, partaking the form of the animal it sustains.

A pulsatory vessel near the gullet propels the yellow blood into a system of fine tubes, that are spread over the walls of the stomach and its rays. Through these walls the blood receives a nutritious liquid, which it carries with it into a network of capillary vessels, widely extended throughout the body, being propelled by the contractile powers of the vessels themselves, and after having supplied the tissues with nourishment, it is carried by tubes to the point from whence it started, to begin a new course. The capillary network passes immediately under a portion of the skin of the star-fish, through which an exchange of the respiratory gases takes place. Besides, the star-fishes breathe the sea-water through numerous conical tubes, that project in patches from the back. Through these tubes, which can be opened and shut, the water is readily admitted into the cavity containing the digestive organs, with which they are in communication. The star-fish slowly distends itself with water, and then gives out a portion of it, but at no regular time. The cavity is never empty of water, and as its lining is densely bristled with cilia, their vibrations keep the vascular surface of the digestive organs perpetually bathed with the respiratory medium.

The star-fishes have a radiating system of nerves suited to their form. A ring of slender nerve-cords surrounds the mouth, from whence three nerves are sent off at the commencement of each ray: two of these, which are filaments, go to the organs in the central disk, while the middle one, which is a great trunk, passes through the centre of the rays, and terminates in a nerve-centre, or ganglion, placed under a coloured eye-speck at their extremity. The structure of the rays, the eye-specks, and the nerve-centres below them, are so similar, that they are merely repetitions of one another; hence no nerve-centre can control the others, but they are all connected by the ring encircling the mouth, which is a common bond of communication. How far the movements of these animals indicate sensation we have not the power to determine, but they feel acutely, for the mouth, the feet, and especially the pedicellariÆ, are highly sensitive, and shrink on the least touch. The eye-specks are probably sensitive to light, and as the star-fishes often feed on putrid matter, they are supposed to be endowed with the sense of smell.

The family of the OphiuridÆ, or Snake Stars, are widely distributed in the ocean. The genus Euryales with branching rays, and that of Ophiura with simple rays, comprising the Brittle and Sand Stars, are abundant in the British seas. In the sand stars there are cavities full of sand at the points from whence the rays diverge, which appear like warts on the surface of the disk. Their rays are exceedingly long, thin, and flexible; they have no central groove nor feet, but they are employed as organs of locomotion and prehension, for by their alternate strokes the sand stars can elevate or depress themselves in the water, creep on the bottom, and by twisting them round objects they can fix themselves, firmly aided by spines or bristles on their edges. The Ophionyx has the addition of movable hooks beneath bristled spines. The rays are bent by the contraction of internal muscles, and extended again by the elasticity of the external leathery coat. The OphiuridÆ, like the Luidia fragilissima belonging to the preceding order, cast off a ray if touched, and even all the five if rudely handled; but they can replace them with as much ease. If only a fragment of a disk remains attached to a ray the whole animal may be reproduced.

The OphiuridÆ have an internal calcareous skeleton or framework, in the form of spicules, scattered in their tissues. They have a capacious mouth with tentacles and ten small chisel-shaped teeth, five on each side, which meet and close the mouth. The mouth is separated from the stomach by a circular muscle that opens and shuts the passage, but no canal diverges from the stomach through the rays. The nervous system and the circulation of the blood are similar to those in the StelleridÆ; and respiratory organs, in the form of from two to four plates, or lamellÆ, project from each of the spaces between the bases of the rays into the central cavity, by which sea-water has free access to bathe the digestive organs and aËrate the blood.

The colour of the star-fishes, as well as of other marine invertebrate animals, seems to be independent of light. The OphiuridÆ that had been living at a depth of 1,260 fathoms in the North Atlantic were coloured, though not a ray of light could reach their dark home, and those dredged up from 100 to 300 fathoms on the coast of Norway were of brilliant hues—red, vermilion, white, and yellow. In general, both plants and animals of the lower kinds become of a sickly white when kept in darkness.

The StelleridÆ are male and female, and form fertilized eggs of an orange or red colour. These eggs are first converted into a mass of cells and then into larvÆ, not radiating symmetrically like their parents, but of a bilateral form, the two sides being perfectly alike and bordered by a ciliated fringe nearly throughout their whole length. These two fringes are united by a superior and inferior transverse ciliated band, and between the two the mouth is placed. A stomach, intestine, and vent are formed; the creatures can provide for themselves, and swim about as independent zooids. A young star-fish is gradually developed by a succession of internal growths, part of the original zooid is retained, and the rest is either thrown off or absorbed; then the star-fishes having lost the power of swimming, crawl slowly away and acquire their full size. There is great diversity in the external form of the zooids of the different genera, as well as in the portion of them retained in the adult star-fish.

Fossil star-fishes have a very wide range. They are found among the earliest Silurian organic forms, but they scarcely bear any resemblance to existing genera. The OphiuridÆ, fished up from the bottom of the North Atlantic, come nearest to them. Five genera are found in the Oolitic formation, all extinct; three genera range from the Lias to the present seas; and five genera belonging to the Cretaceous period are represented by living species.

Echinodermata CrinoÏdea.

The Crinoid Echinoderms, or Stone-Lilies, are like a tulip or lily on an upright stem, which is firmly fixed to a substance at the bottom of the sea. During the Jurassic period, miniature forests of these beautiful animals flourished on the surface of the Oolite strata, then under the ocean. Myriads of their fossil remains are entombed in the seas, and extensive strata of marble are chiefly composed of them. Their hollow joints are known in several parts of England as wheel stones, and as St. Cuthbert’s beads on the Northumbrian coast, in honour of the patron saint of Holy Island, where they abound. The CrinoÏdea are of two kinds: the Encrinites, which chiefly flourished in the PalÆozoic period and are now represented by a minute species (Rhizocrinus Lofotensis) lately discovered on the coast of Norway by Professor Sars, have a smooth, cylindrical, jointed stem; and the Pentacrinites, which began at the Lias, and have a five-sided jointed stem, the present representative of which is the Pentacrinus caput-MedusÆ, found in the West Indian seas.

The hollow, five-sided, calcareous, jointed stem of the living Pentacrinite is filled with a spongy substance, and supports a cup on its summit, containing the digestive organs, mouth, and tentacles of the animal. The cup is formed of a series of calcareous plates, and from its margin five long many-jointed rays diverge, each of which is divided into two-jointed branches. Groups of curled filaments, called cirri, are placed at regular distances from the bottom of the stem to the extremity of the rays, while, on the opposite side of the rays, there are groups of feathery objects called pinnÆ at each joint. Food is caught by the tentacles and digested by the stomach and viscera at the bottom of the cup, from whence vessels diverge through a system of canals in the axes of the rays, pinnÆ, and down the stem, all of which convey sea-water mixed with nutritious liquid, for the nourishment of the animal.

The genus Comatula are star-fishes, believed to have alternately a fixed and a free state. Mr. J. V. Thomson discovered that the Pentacrinus EuropÆus is merely the fixed state of a Comatula. These star-fishes have pairs of pinnÆ placed at regular distances along their long-jointed rays, and in the pinnÆ sacs containing eggs are placed as far as the fifteenth or twentieth pair. The eggs yield active ciliated larvÆ, which attach themselves in the form of flat oval disks to corallines and sea-weeds. By degrees they develop a stem, about three-fourths of an inch high, with twenty-four distinct joints. Its expanded top bears five sulphur-coloured bifurcating rays with their pinnÆ and dorsal cirri. A mouth is formed in the centre with its tentacles, and a lateral prominent vent. The actual change of a Pentacrine into a Comatula has not been seen, but as the small Pentacrinites disappear in September, at which season the ComatulÆ appear, it is believed that when full grown the top of the fixed Pentacrinite falls off and becomes a Comatula, which swims backwards with great activity by striking the water alternately with its long rays. The Pentacrinus caput-MedusÆ, which is fixed by its stem to sea-weeds and zoophytes, forms a most beautiful object for the lower magnifying powers when viewed in a fluid by a strong refracting light.

Echinodermata EchinoÏdea.

The family of EchinidÆ, commonly known as Sea-Eggs or Sea-Urchins, have a beautiful but complicated structure. The calcareous shell of an Echinus is a hollow spheroid with large circular openings at each pole. In the larger of the two, called the corona, the mouth of the animal is situated; in the lesser circle the vent is placed. The spheroid itself is formed of ten bands extending in a meridional direction from the corona to the lower ring; that is, from one polar circle to the other. Each band consists of a double row of pentagonal plates increasing in size from the poles to the equator, nicely dovetailed into one another, and the bands are neatly joined by a zigzag seam. Every alternate band is perforated by a double series of minute double holes for the passage of the tubular feet of the animal. The five perforated or ambulacral bands have rows of tubercules parallel to the series of feet holes, supporting spines movable in every direction. The five imperforated bands are characterized by a greater number of spines, but there are none within the polar circles. The spines may be long rods, or merely prickles, or stout, club-shaped bodies, according to the genera.

The microscopic structure of the shell of the Echinus is everywhere the same; it is composed of a network of carbonate of lime, with a very small quantity of animal matter as a basis. In general, the network extends in layers united by perpendicular pillars, but so arranged that the open spaces, or meshes, in one layer correspond to the solid structure in the next.

The spheroid of the Echinus is covered with spines, and both outside and inside by a contractile and extensile transparent membrane, which supports the shelly plates at the poles, and dips between the bands but does not penetrate them. Its extensile nature admits of the addition of calcareous matter to the edges of the plates when the animal is increasing in size. The membrane lining the interior of the shelly globe is tough; it encloses the digestive organs, and forms a muscular lip to the mouth, which is armed with five triangular, sharp-pointed, white teeth, and surrounded by five pairs of pinnate tubular tentacles. The outer margin of the lip is fringed with a circle of snake-headed pedicellariÆ visible to the naked eye.

The five teeth, whose sharp tips meet in a point when closed, are triangular prisms, the inner edge is sharp and fit for cutting. Each tooth is planted upon a larger triangular socket, two sides of which are transversely grooved like a file, and as these two sides are in close contact with the sides of the opposite socket, the food when cut by the small teeth is ground down by the sockets, and a salivary secretion finishes the mastication. The sockets of the teeth are connected by ten additional solid pieces, placed two and two between them, which completes the pyramidal apparatus called Aristotle’s lantern; it consists of forty solid calcareous pieces arranged in fives, and moved by forty muscles attached to five calcareous ridges, and five arches near the internal edge of the corona.

Five pairs of these muscles when acting together protrude and retract the teeth; when acting separately they draw them to one side or to the other; five pairs separate the five teeth, five pairs shut them, and the remaining five pairs work the bruising machine. The masticated food passes through a short gullet into the stomach, where it is digested, and the indigestible part is carried by an intestine to the vent in the smaller polar circle.

The smaller polar circle is formed of ten triangular plates, five are attached to the bands containing the feet holes, and five to the intermediate bands. The last five are perforated, and are the reproductive plates: the other five are also perforated for the discharge of the liquid that moves the tubular feet, and which, after having circulated in the body, is no longer of use. In five of these polar plates there are red specks, the rudiments of eyes, the only organs of sense these creatures seem to possess except that of touch and probably smell. The nervous system is a slender, equal-sided pentagon round the gullet, from the sides of which five nerves are sent to the muscles of the mouth, and others, extending along the ambulacral or feet bands, end in nerve-centres under the eye-specks.

The mechanism for extending and retracting the feet by a liquid, is the same with that in the star-fishes, but the pores which admit the liquid into the feet are double. The tubular feet swell at their extremity into a fleshy sucker, within which there is a thin glassy reticulated rosette (fig. 139), of which fig. 140 is a highly magnified segment. It is perforated in the centre by a large round opening. The sea-urchins can stretch their feet beyond the spines, and by means of the suckers they can attach themselves even to smooth objects, or aided and directed by their spines they roll themselves along with a rotatory motion head downwards.

The circulation of the bright yellow blood is like that of the star-fishes. It is aËrated both internally and externally. The external respiratory organs are short, branched, and highly vibratile bodies attached in pairs to the oval extremities of the fine imperforated bands.

There are pedicellariÆ scattered among the spines of the sea-urchins which are in constant motion, protruding themselves beyond the spines and withdrawing again, snapping their pincers, and grasping firmly anything that comes within their reach, or that is presented to them. The pedicellariÆ vary much in form and position in the different genera of the EchinidÆ; but they invariably consist of a long, slender, calcareous stem, and generally tripartite head, the whole coated with a gelatinous fibrous transparent substance. The head of the Pedicellaria globosa is a formidable weapon; at the apex of each of its three serrated and toothed blades there is a strong sharp spine directed horizontally inwards, so that the three spines cross each other when the blades close, which they do so energetically that nothing could escape from such a grasp. The pedicellariÆ are curious microscopic objects; they are extremely irritable, and although their use is unknown, they must be essential to the well-being of the animals, since hundreds are scattered over their shells.

The spines of the EchinidÆ vary in shape and structure in the different genera and species. Those of the Scutella form merely a velvety pile. On the common sea urchin the spines are simple, and shed twice in the year; those on the Amphidetus are both club and spoon-shaped; and, on the Cidaris, they are large formidable clubs moved by a ball and socket. All the spines, whatever their form may be, are moved in that manner; for there are little tubercules on the surface of the shell on which a cup at the bottom of the spines is pressed down by the muscular skin which covers the shell and spines, and by its contractile power it enables the animal to move the spines in any direction.

The microscopic structure of the calcareous spines is often beautifully symmetrical. Those of the Acrocladia mamillata consist of concentric alternate layers of network and sheaths of pillars; so that a section of the spine perpendicular to its axis exhibits a succession of concentric rings like those of an exogenous tree. The cup at the bottom of the spine is very dense network, and the last of a sheath of encircling pillars form the ribs, sometimes seen on the exterior of the spines.

The spines of the Echinus miliaris, of which fig. 141 represents the segment of a section highly magnified, are fluted columns of calcareous glass, the grooves of which are filled with solid glassy matter curved on the exterior. The innumerable hair-like objects attached to the shells of some of the EchinidÆ, the almost filamental spines of others, and the pedicellariÆ themselves, are formed of a regularly reticulated substance. When the EchinidÆ are stripped of their spines and all their appendages, their shells show 2,400 plates united with the symmetry of a tesselated pavement.

The EchinidÆ are male and female, and the eggs are excluded through the five perforated productive plates at the posterior end of the shell. According to the observations of Prof. Fritz MÜller the embryo, soon after issuing from the egg, takes a form represented (magnified) in fig. 142.

All parts of this creature, which is called a Pluteus, are strengthened by a framework of calcareous rods tipped with orange colour, all the rest being transparent and colourless. It swims freely, back foremost, by means of its cilia.

While in this active state a circular disk (c, fig. 142), covering the stomach (b, fig. 142), appears within it, which gradually expands, and sends through the skin of the Pluteus spines, pedicellariÆ, and tubercules, ultimately developed into hollow feet. Then the feet are pushed out and drawn in, the pedicellariÆ (D, fig. 143) snap their pincers; and while the half-formed Echinus is making these motions within the Pluteus, the mouth and gullet of the Pluteus itself are in constant activity; and, while it swims about, the unformed Echinus within it gets a globular shape, the shell is formed, and when the Echinus is complete, the rest of the Pluteus is thrown off, and the young animal rolls away.

The free swimming larval zooids of the Echinodermata are generally hyaline, and some are phosphorescent. The Pluteus is also the larval zooid of the ophiurid star-fishes; they may be seen in great numbers on the surface of the sea in August and September. The young star-fish is formed in them by a process analogous to that described. The motions of the EchinidÆ are reflex; nothing indicates volition.

The fossil EchinidÆ first appeared in the lower Ludlow limestone, and attained their maximum in the Cretaceous strata. A species of Diadema, with annulated hollow spines, common in the Chalk, still exists. Numerous species of the genus Clypeaster, remarkable for their flattened form, and known as lake urchins, are peculiar to the Tertiary strata and existing seas; and, lastly, five species of SpatangidÆ, heart-shaped urchins, which lived in the Tertiary periods, still exist. In consequence of the porous texture of the solid calcareous parts of the EchinidÆ, their fossil remains are commonly impregnated with pyrites or silex, without altering their organic structure, so that they exhibit a fracture like that of calcareous spar.

Echinodermata HolothuroÏdea.

The HolothuridÆ, or Sea-Cucumbers, are of a higher organization than the preceding Echinoderms. They are soft, worm-shaped, five-sided animals, covered by a flexible, leathery integument or skin, in which are imbedded a vast multitude of microscopic calcareous plates of reticulated structure. The mouth, which is placed at one end of the animal, is surrounded by ten bony plates forming a lantern, analogous to that of the Echinus; they support branching, tubular, and retractile tentacles, which encompass the mouth like a star. The tentacles are connected with sacs at their bases, and are extended and retracted by the injection of a watery liquid contained in them. Innumerable tubular, suctorial feet, precisely similar to those of the Echinus, are protruded and retracted through corresponding pores in the skin of the animal by a watery liquid, in sacs, at their bases. The water is supplied by a system of canals connected with an annular reservoir round the top of the gullet, which is supplied with water by a bottle-shaped bag at the mouth.

Besides transverse muscles, five pairs of muscles attached to the lantern at the mouth, extend throughout the whole length of the animal. Nerve-chords from the ring at the gullet accompany these, and such is the irritability of this muscular system, that the HolothuriÆ eject their viscera when alarmed or caught; but they have the power of reproducing them: sometimes they divide their whole body into parts.

The respiratory organs are two very long and beautifully arborescent tubes veined with capillary bloodvessels. The circulation of the blood is similar to that of the star-fishes, but more complicated.

The minute calcareous particles scattered independently in the tough leathery skin of the HolothuridÆ remain as fine dust when the flesh is dissolved and washed away; but, upon microscopic observation, Mr. Gosse found that the forms of these particles are remarkable for elegance, regularity, and variety of structure, but that the normal form is an ellipse of open work built up of five pieces of a highly refractive, transparent, glassy material, having the shape of dumb-bells.

The HolothuriÆ found under stones at low spring tides, on the British coasts, are small; those dredged up from deep water are five or six inches long, and not unlike a well-grown warty cucumber; they do not form an article of food in Europe, but they are highly esteemed by the inhabitants of the Indian Archipelago and in China, where many shiploads of the trepang are imported annually. It is a species that swarms in the lagoons of the coral islands, the reefs of the coral seas, and at Madagascar. Some species are two feet long, and six or eight inches in circumference.

The order of the HolothuridÆ form eggs like all the other Echinoderms; the larval zooid has the same form as that of the star-fishes, and changes its form twice, while the members of the Holothuria are forming within it; at last they combine with those of the zooid, and no part is cast off.

The SynaptidÆ are five-sided creatures, similar in structure to the HolothuriÆ, though more worm-like. The whole order, which consists of the two genera of Synapta and Chirodota, have twelve calcareous plates round the mouth, five of which are perforated for the passage of the vascular water canals, which convey the liquid for the protrusion of the feet.

The calcareous particles imbedded in the skin of the genus Synapta are anchor-shaped spicules fixed to elliptical or oval plates, (fig. 144). The plates are reticulated and sometimes leaf-shaped, and the flukes of the anchors are either plain or barbed. All the anchors are fixed transversely to the length of the animal, lying with great regularity in the interspaces of the longitudinal muscular bands. Sometimes a thousand anchors are crowded into a square inch, each elegant in form, perfectly finished, and articulated to an anchor-plate, whose pattern as well as that of the anchor itself is characteristic of the species to which it belongs. In the Synapta digitata, which has four fingers and a small thumb on each of its twelve oval tentacles, the anchors are but just visible to the naked eye;^[36] in all the other species they are microscopic. Besides the anchors, the skin of the genus Synapta contains innumerable smaller particles, ‘miliary plates,’ which are crowded over the muscular bands. The muscular system of the Synapta digitata is so irritable that, on being touched, it divides itself into a number of independent fragments, each of which keeps moving for a time, and ultimately becomes a perfect animal like its parent. Specimens of this Synapta have been found on the southern coasts of England and in the West of Scotland, but the genus is rare, although containing several species in the British seas; it is more common in the Adriatic; but they cannot be compared, as to size, with the great Synapta of Celebes, which is sometimes a yard in length, and is known among the natives as the Sea Serpent.

The calcareous particles imbedded in the skin of the allied genus Chirodota are wheel-shaped when viewed with a microscope (fig. 145). One species is British, but they are mostly inhabitants of warm seas. In Chirodota violacea, a Mediterranean species, the skin is full of groups of broad thin hyaline wheels lying upon one another and connected by a fine thread. The wheels have five or six flat radiating spokes.^[37] The wheels are exceedingly small in the Chirodota lÆvis, and are arranged in groups; in the C. myriotrochus they are imbedded in myriads, as the name implies.

Echinodermata SipunculidÆ.

The SipunculidÆ, which form the last order of the Echinoderms, consist of several genera of marine worm-shaped animals which burrow in the sand, and form a link between the HolothuridÆ and the true sea-worms. They have no calcareous particles in their flexible skins, nor have they any tubular feet, or special respiratory organs, but a vascular liquid is kept in motion in the internal cavity by the cilia with which it is lined. The mouth of the Sipunculus is a kind of proboscis with a circular fringed lip and two contractile vessels, supposed to serve for raising the fringes. An alimentary canal extends to the end of the animal, turns back again, and the intestine ends in a vent near the mouth, so that the creature need not leave its burrow and expose itself to enemies in order to eject the refuse of its food. The locomotive larval zooids from the rose-coloured eggs undergo two metamorphoses; at last the young Sipunculus unites with the zooid, and no part is thrown off.