Still passing up the scale of animal life, we now come to the Echinodermata—the other sub-kingdom which we have already referred to as forming, with the Coelenterates, the old division of Radiata. The term Echinoderm signifies ‘hedgehog skin,’ and is applied to the group on account of the fact that the majority of its species possess a skin that is either distinctly spiny, or exhibits numerous more or less defined prominences. This skin is also supported and hardened by the deposit of little plates or spicules of carbonate of lime, all joined together so as to form a kind of scaffolding or ‘test’ for the protection of the animal; and this secretion of carbonate of lime is not always confined to the outer skin, for, in some cases, it occurs in the walls of the internal organs as well.

Most of the animals of this sub-kingdom display a regular radiate symmetry; that is, the parts of their bodies are arranged regularly round a common axis, and the arrangement is usually a five-fold one, as may be observed in the case of the common Five-fingered Starfish of our coasts (see Plate IV.), and it is worthy of note that this radiate disposition of parts is not merely external, but that, as in the case of anemones and jelly-fishes, it also obtains within, and determines the arrangement of the internal organs. Further, although this radiate symmetry characterises the adult animals of the group we are considering, yet some show a tendency towards bilateral symmetry (parts arranged equally on two opposite sides of a common axis), while this is the rule, rather than the exception, with the early stages or larvÆ of these creatures. Observe, for instance, the larva of the common Brittle Starfish, the adult of which species exhibits an almost perfect radiate symmetry, and we see something more than a mere trace of a two-sided disposition.We have not to look far into the structure of any typical echinoderm to see that it is a distinct advance on the anemones in the matter of organisation. To begin with its digestive system—this consists of a tube having no communication with the general body-cavity, but remaining quite distinct throughout its length, with both ends communicating directly with the exterior. Its nervous system also is more highly developed, for it has a well-formed ring of nerve matter round the mouth, from which pass two or three systems of nerve fibres, each system having its own special function to perform. The sense organs, however, do not appear to be well developed, though there exist certain ‘pigment spots,’ in which nerve fibres terminate, and which are supposed to serve the purpose of eyes.

One of the most interesting features in connection with the echinoderms is undoubtedly the structure and function of the apparatus for locomotion. Examine a live sea urchin, or the common five-rayed starfish, in a rock pool or aquarium, and it will be seen to possess a large number of soft, flexible, and protrusible processes, each of which terminates in a little sucking-disc that enables the animal to obtain a good ‘foot-hold;’ and, having fixed itself on one side by means of a number of these little ‘feet,’ it is enabled, by the contraction of certain muscles, to pull itself along.

The little feet we are examining are really tubes filled with water, and capable of being inflated by the injection of water into them from within the body of the animal. Each one communicates with a water tube, several of which (usually five) radiate from a circular canal of water that surrounds the mouth. This circular canal does not communicate with the mouth, but with a tube, known as the ‘stone canal’ because of the carbonate of lime deposited within its walls, that opens at the surface of the body on the opposite side, and is guarded at the orifice by one or more perforated plates through which water gains admission. Thus the animal can fill its ‘water system’ direct from the sea, and, by the contraction of muscles that surround the main canals, force this water into the little ‘tube-feet,’ causing them to protrude and present their sucking-discs to any solid object over which it desires to creep. We may observe, however, that some of the little protrusible tubes have no sucking-discs, and probably serve the purpose of feelers only; also, that while these tube-feet are the principal means of locomotion in certain species, in others the movements of the body are performed almost exclusively by the five or more rays that extend from the centre of the animal, and which are readily curved into any desired position by the action of well-developed muscles.

All the echinoderms come within the domain of the marine naturalist, for no members of the sub-kingdom are inhabitants of fresh water; and it is interesting to observe that, unlike the animals previously described, none of them live in colonies.

A general examination of the various starfishes to be found in our seas will show that they may be divided into three distinct groups. One of these contains the pretty Feather Stars, which are distinguished by their long and slender ‘arms,’ usually ten or more in number, each of which bears a number of pinnules that give it quite a feathered appearance. The second includes the Brittle Stars, possessing five slender arms that are jointed to the small, flattened, central disc, and which are so named on account of the readiness with which the animal falls to pieces when alarmed or disturbed; and the third is formed by the remaining five-rayed stars, the arms of which, instead of being jointed to, are continuous with, the centre of the body.

All these starfishes have a leathery skin, supported and hardened by a framework of calcareous plates, and presenting a number of hard ridges or spines. In addition to the system of water tubes already mentioned as characteristic of the echinoderms, they also possess a second circular vessel round the mouth, from which a number of vessels are distributed to the walls of the digestive tube. These, however, are bloodvessels, and are directly concerned with the nutrition of the body. Some, also, have imperfectly developed eyes at the ends of the arms or rays.

Contrary to what one would expect after watching the somewhat sluggish movements of starfishes, they are really very voracious creatures, attacking and devouring molluscs and small crustaceans, sometimes even protruding their stomachs to surround their prey when too large to be passed completely through the mouth; and they are also valuable as scavengers, since they greedily devour dead fishes and other decomposible animal matter.

Feather Stars differ from other starfishes in that they are stalked or rooted during one portion of their early life. At first they are little free-swimming creatures, feeding on foraminifers and other minute organisms that float about in the sea. Then they settle down and become rooted to the bottom, usually in deep water, at which stage they are like little stalked flowers, and closely resemble the fossil encrinites or stone lilies so common in some of our rock beds, and to which they are, indeed, very closely allied. After a period of this sedentary existence, during which they have to subsist on whatever food happens to come within their reach, they become free again, lose their stalks, and creep about by means of their arms to hunt for their prey.

The commonest British species of these starfishes is the Rosy Feather Star (Antedon rosaceus); and as this creature may be kept alive in an aquarium for some considerable time without much difficulty, it will repay one to secure a specimen for the observation of its habits. It is not often, however, that the Feather Star is to be found above low-water mark, its home being the rugged bottom under a considerable depth of water, where a number usually live in company; but there is no difficulty in obtaining this and many other species of interesting starfishes in fishing towns and villages where trawlers are stationed, for they are being continually found among the contents of the net.

Although the Feather Star can hardly be described as an active creature, yet it will cover a considerable amount of ground in the course of a day, creeping over rocks and weeds by means of its arms, which are raised, extended, and again depressed in succession, each one thus in turn serving the purpose of a foot. These arms are capable of being moved freely in any direction, as are also the little more or less rigid pinnules appended to them. The latter are bent backwards on an extended arm that is being used to pull the animal along, so that they form so many grappling hooks that hold on the bottom; and then the arm in question is bent into a curve by the contraction of its muscles, thus dragging the body forward. The arms on the opposite side of the body are also used to assist the movement by pushing it in the same direction, and this is accomplished by first bending the arms, and then, after curving the pinnules in a direction from the body, again extending them. Other movements of the feather star are equally interesting. Thus, the manner in which it will suddenly extend its arms and apply its pinnules to the surface on which it rests in order to obtain a good hold when alarmed, and the way in which it apparently resents interference when one of the arms is touched, are worthy of observation. The arms themselves are readily broken, and will continue to move for some time after being severed from the body, but the loss to the animal is only temporary, for a new arm grows in the place of each one that has been broken off.

This tendency to break into pieces is much greater in the Brittle Stars, as might be expected from their popular name; and is, in fact, such a marked characteristic of the group that it is not by any means an easy matter to obtain a collection of perfect specimens. They will often snap off all their arms, as if by their own power of will, when disturbed or alarmed, and even when removed from their hold without injury, they will frequently break themselves into pieces if dropped into spirit or in any way subjected to a sudden change of conditions.

The tube-feet of Brittle Stars are very small and are not provided with suckers, but are very sensitive, serving the purpose of feelers; also, having thin, permeable walls, they probably play a large part in the process of respiration. Both arms and disc are hardened by a dense scaffolding of calcareous plates; and not only are the former attached to the latter by a well-formed joint, but the arms themselves are constructed of a number of segments that are held together by a kind of ‘tongue and groove’ joint. Round the mouth are a number of tentacles that are kept in constant motion with the object of carrying the food towards it, and of holding the larger morsels while the act of swallowing is progressing.

The various species of Brittle Stars live among the rocks and weeds, chiefly in deep water, where they move about by means of the muscular contraction of their arms, the disc being raised on the curved arms as the animal proceeds. Some species are to be found between the tide-marks, and especially abundant on the south-west coast are two small species that live among the tufts of coralline weeds, sometimes so crowded together that dozens may be taken from a little patch of coralline only two or three inches square. These have such small discs, and such slender arms, and are, moreover, so well concealed by their colouring, which closely resembles that of the weed-tuft they inhabit, that they are only to be detected by close inspection.

The remaining division of the starfishes, sometimes distinguished by the name of Common Stars, possess five arms or rays, which may be either long or short, and which are not jointed with the central disc, but continuous with it; that is, there is no sharp line of demarcation between arm and disc. One or two species are well known to all frequenters of the sea-side, but the majority of them inhabit deep water, where they creep about over the rocks and weeds, obtaining their food from the bed below them.

If we examine the common five-finger star that is so often stranded on the beach, and so frequently found in rock pools between the tide-marks, we see that each arm has a large and conspicuous groove running along its centre on the under side, and on each side of these are the rows of tube-feet, arranged in such a manner that they have suggested the appearance of an avenue of trees on each side of a garden walk, and have consequently earned the name of ambulacrum. These tube-feet may be protruded for some distance; and, being provided with suckers that possess considerable clinging power, they form the principal means of locomotion.

Put the starfish in the aquarium, or in a tidepool by the sea, and you will find it very interesting to observe how the animal progresses, while some idea of the clinging power of the tube-feet may be ascertained by allowing the animal to creep over the submerged hand.

The movements of the tube-feet may also be seen to advantage when the starfish is laid upside down in a pool, and, what is still more interesting, the manner in which the animal turns itself over. To do this it will first bend the tips of one or two of its arms so as to bring the suckers against the ground; and then, aided by the pulling action of these, it will gradually bring other suckers into a similar position till, at last, the whole body has been turned over. Some of our common starfishes have rays so short that they may be termed angles rather than arms, and these are unable to turn their inverted bodies by the gradual method just described. They generally raise their bodies on the tips of three or four of the rays, assuming somewhat the form of a three-or four-legged stool, and then, bending the remaining one or two arms over the body, they alter the position of the centre of gravity till eventually the body topples over to the desired position.

Some of the common five-rayed stars have no suckers on their tube-feet, and consequently have to creep by means of the muscular contractions of their arms; and several of them are like the brittle stars in breaking up their bodies when irritated or seized. This latter peculiarity will account for the frequency with which we come across animals with one or more rays smaller than the others, the smaller rays being new ones that have been produced in the place of those lost. Again, we sometimes meet with such monstrosities as a five-rayed star with six or more rays, some smaller than others, the smaller ones representing two or more that have grown in the place of one that has been lost; or a starfish with branched or forked arm, illustrating the tendency to produce a new arm even when the original one has been only partially severed.

A close observation of a starfish in water may enable us to detect a number of little transparent processes standing out between the prominences of the rough skin of the upper surface. These are little bags filled with fluid, formed of such thin walls that gases can readily pass through them, and are undoubtedly connected with the process of respiration. Also, on the upturned extremity of each arm a red spot may be seen; and this from the nature of its structure, and from its association with the nervous system, has been regarded as a rudimentary eye.

On the upper side of the disc one may also observe a more or less conspicuous spot of variable colour, on one side of the centre. It is a plate, finely perforated, covering the outer extremity of a short canal which communicates with the system of water tubes that were described in the earlier part of this chapter. It is, in fact, the entrance through which water is admitted into the central ring round the mouth, and from this into the radial water tubes that run through each arm of the starfish to supply the tube-feet. The short tube referred to is always filled with sand, and thus the water that enters into the water-vascular system is filtered before it reaches the circular vessel. It is interesting to note, in this connection, that here is one respect in which the radiate symmetry of the starfish is broken, there being only one entrance, and that not a central one, by which water is distributed into the five rays.

Of course, when the ray of a starfish has been broken off the water vessel or vessels that it contained are destroyed, as is also the prolongation of the stomach, in the form of a long, blind tube, that extended into it. But no inconvenience attaches itself to this loss, for the starfish has the power of reproducing even its lost viscera, as well as any of the five rays of the body that may be broken off.

We must briefly refer to one other feature of the common star, viz. the possession of small prehensile organs around the mouth. These are little spines, the extremities of which are movable, and take the form of little pincers by means of which the animal can hold its prey.

When it is desired to preserve starfishes for future study, immersion in diluted spirit or a solution of formaldehyde will answer all purposes, the soft parts being thus preserved as well as the harder structures; but it is usual to preserve them in a dry state when they are required merely for purposes of identification, as is usually the case with the specimens in an ordinary museum collection. In the latter case it is advisable to put the starfishes in strong spirit for a few days, changing the spirit if several specimens are put together, and then drying them as quickly as possible in the open air.

We have now to consider the Sea Urchins or Sea Eggs, which are readily known by the hedgehog-like covering of hard spines. Externally they appear as globular or heart-shaped bodies, the surface entirely hidden by spines except, perhaps, the mouth on the under side, which is provided with an apparatus for mastication. If alive, and in the water, one may notice that the animal creeps along the bottom, mouth downwards, moving itself either by means of its moveable spines, or by soft tube-feet resembling those of starfishes, that are protruded between the spines, or by both combined; and the movements of its masticating organ may be seen by observing the animal through the side or bottom of a glass vessel of sea water. The last-named organ is surrounded by an area of soft skin, and is not present in all species.

A closer examination of the common globular urchin will show that it is wonderfully constructed. Even the spines, which are in themselves uninteresting objects to the naked eye, are most beautifully formed, a transverse section revealing a radiate or reticulated structure when viewed through the microscope. Each spine has a concave base which fits on a little tubercle of the calcareous shell or test that covers the body of the animal, forming a perfect ball-and-socket joint, and is capable of being moved in any direction by means of small muscular bands.

On removing the spines the shell is seen to completely enclose the animal with the exception of the mouth, with its masticatory apparatus, and the small area around it which is covered by the uncalcified skin just referred to.

At the very top of the shell, exactly opposite the mouth, there is a small plate perforated by the extremity of the digestive tube. Round this are five angular plates, each perforated by the ducts of the ovaries or egg-producing glands, but one of these is enlarged and further perforated, that it may serve the second purpose of allowing water to enter the system of water tubes that supply the tube-feet, and thus corresponds exactly with the plate already noticed on the upper surface of the starfish. Between these are five smaller plates, each with a rudimentary eye that receives a fine nerve-thread.

The remaining and greater portion of the shell of the urchin is composed of ten radiating segments, each of which is made up of two rows of flat angular plates united at their edges. Five of these segments, arranged alternately with the others, are perforated by numerous holes, through which the tube-feet of the urchin are protruded, while the remainder are imperforate; and all ten plates bear the little hemispherical processes to which the spines are jointed.

One of the most interesting features of this urchin is undoubtedly its complex and wonderful masticating system. There are five teeth, symmetrically arranged, and all pointing towards the centre of the mouth. Each is attached to a wedge-shape jaw, made up of several pieces, and the whole apparatus is attached by ligaments to loops in the interior of the shell, and is moved by no less than thirty distinct muscles. The complete system may be readily dissected out, and is well worthy of study and preservation. (The harder portions of the system may often be found in the interior of the empty shell of an urchin after the softer structures of the body have decayed away.)

An interesting dissection of the globular urchin may also be made by cutting completely round the shell with a pair of sharp-pointed scissors midway between the mouth and the apex, and then separating the upper and lower halves, as shown in fig. 114. In this way the whole of the digestive tube, with its numerous curves, may be traced from the mouth to the anus at the opposite pole. The water-vessels that supply the tube-feet in the regions of the five perforated plates may also be seen, as well as the ovaries or egg-producing organs and the bases of the five jaws with their complicated system of muscles.

A little acquaintance with the commonest of the British sea urchins will show that they may be divided into two well-defined groups, one containing the globular or subglobular forms, of which the common sea urchin or sea egg (Echinus sphÆra) above described, is a type, as well as the pretty little Green Pea Urchin (Echinocyamus pusillus), and the little Purple-tipped Urchin (Echinus miliaris), which is found principally on the west coast of Scotland; while the second group is formed by the less symmetrical Heart Urchins, which differ from the others in several interesting particulars of structure and habit.

These heart urchins (Plate IV., fig. 4) are covered with short, delicate spines which are not much used for purposes of locomotion, the animals moving from place to place almost entirely by means of their tube-feet, while the globular urchins travel principally by their spines, which are stouter and more freely moved on well-formed ball-and-socket joints. Also, while in the globular species the perforated plates that admit of the protrusion of the feet are arranged with a perfect radiate symmetry, those of the heart urchins are confined to one side of the shell; and the digestive tube, which in the former terminates in the pole opposite the mouth, in the latter ends close to the mouth itself. Further, the heart urchins do not possess any kind of dental apparatus.

The habits of sea urchins are interesting, and may be watched in the aquarium, where the movements of the spines and of the tube-feet may be seen perfectly. Some species are very inactive, living in holes and crevices, or under stones, and seldom move from their hiding-places, while others travel considerable distances. The former have generally no eyes, and, instead of seeking their food, simply depend for their subsistence on the material carried to them by the movements of the water; while the latter possess visual organs similar to those observed in certain starfishes. Some species also protect themselves from their enemies when in the open by covering their bodies with sand, small stones, shells, or weeds, and thus so perfectly imitate their surroundings that they are not easily detected. The feet that are used for purposes of locomotion terminate in suckers resembling those of the common five-fingered starfish, and have considerable clinging power, but some have either very imperfectly developed suckers or none at all, and are probably used as feelers only.

Sea urchins, like their allies the starfishes, generally inhabit deep water beyond low-water mark, where they often exist in enormous numbers, feeding on both animal and vegetable substances; but some species are often to be met with between the tide-marks, where they may be seen under stones, and frequently half hidden in mud. The globular species occur principally on rocky coasts, but the heart urchins are more commonly dredged from banks of sand or mud that are always submerged.

The life-history of urchins closely resembles that of starfishes, for the young are free-swimming creatures of an easel-like form, and during this early larval existence their bodies are supported by a calcareous skeleton.

We will conclude our short account of the British echinoderms with a description of the peculiar Sea Cucumbers, which belong to the division Holothuroidea. These creatures are so unlike starfishes and urchins in general appearance that the uninitiated would hardly regard them as close relatives. The body is, as the popular name implies, cucumber-shaped, with the mouth at one end, and the general aspect is wormlike. There is, however, a radiate symmetry—a five-fold arrangement of parts, though not so regular as in most echinoderms. Running lengthwise along the body are five rows of tube-feet, but only two of these are well developed and terminate in functional suckers; and, as might be expected, the animal crawls with these two rows beneath it. The feet are outgrowths of a system of water tubes similar to that of the urchin, there being a circular tube round the mouth, from which branch five radial tubes, one for each row.

The mouth of the sea cucumber is surrounded by plumed tentacles which can be retracted at will, and which are used in capturing the smaller living things that form its food. Like the earthworm, it will often swallow large quantities of sand, from which it digests the organic matter contained.

The body-wall of the Holothuroidea is strong and muscular, and is strengthened by the presence of numerous spicules of carbonate of lime, often in the form of little anchors, wheels, and crosses, while the outer surface is rough and slimy, and often of a colour so closely resembling the surroundings of these animals that they are not easily observed. This feature is one of great value to the creatures, since they have no means of defence from their enemies, and seem to owe their safety entirely to their protective colouring.

There are several species of sea cucumbers on our coasts, but all inhabit deep water and are seldom to be seen above low-water level. They are, as a rule, easily obtained from fishermen, who will bring them in when requested to do so. Live specimens may be kept for a considerable time in the indoor aquarium, and seem to prefer a rocky bottom on which they can hide under stones at times, and a bed of sand on which they will occasionally crawl. They will readily devour small molluscs and crustaceans, and will partake of dead organic matter in a partially decomposed state.The following tabular summary of the classification of Echinoderms may possibly be of use for reference:—