CHAPTER III.

_STAR-FISHES, SEA-URCHINS, ETC._

The somewhat varied assemblage of marine animals familiarly known as STAR-FISHES, FEATHER-STARS, BRITTLE-STARS, SEA-URCHINS, and SEA-CUCUMBERS all agree structurally with one another and differ from all other living organisms in several conspicuous features. Prominent among these is the circumstance that their protecting skin is more or less extensively impregnated externally and strengthened internally with calcareous elements which take the form of plates and spines and spicules.

The COMMON SEA-URCHIN may be cited as an example in which these calcareous elements attain their maximum development. The subspherical box-like case or shell, wherein all the vital organs are enclosed and safely protected, is a common object of the seashore, and, empty and denuded of its external coating of prickly spines, familiarly known as a "sea-egg." Examined closely, this shell is found to consist of a series of calcareous plates, which dovetail or fit together in juxtaposition with the utmost nicety. The surface of the shell is studded throughout with rounded hemispherical knobs, those of a larger size having a very distinctly symmetrical plan of distribution. These rounded knobs are the bases of attachment of the spines, which radiate at all points from the surface of the shell when the animal is alive. It will be further recognised on a nearer examination that the walls of the shell are pierced on a definitely symmetrical pattern with minute perforations, such perforations being most distinctly visible on the inner surface of the shell. These minute punctures are the apertures through which in life the delicate tubular locomotive organs, or so-called "feet," are thrust out and retracted. The majority of these tubular organs terminate in a circular sucking-disk, wherewith, collectively, the urchin is able to adhere to and travel over the surface of the smoothest rock, or even up the glass walls of an aquarium. In the empty beach-gathered urchin-shell a circular hole may be observed at the two opposite poles, the one in the centre of the lower and flatter surface being the larger of the two. It is within this lower and larger one that the mouth, with its complex apparatus of teeth, is suspended. The membranous disk which covers the upper and smaller circular aperture in the living animal is perforated centrally by the vent, and around it are grouped the eye-spots and sundry excretory apertures.

A noteworthy feature associated with the greater portion of the structural details of the sea-urchin which have been enumerated is the dominance of the number five in the constituent elements. It is found, for instance, that the perforated areas through which the tube-feet are protruded form, as with the petals and other elements of many flowers, five symmetrically corresponding segments. The dental apparatus comprises five equivalent tooth-like structures, and there are five eye-spots and five excretory apertures at the upper pole. This particular number, with multiples of the same, is furthermore characteristic of all the typical members of the class. Thus, in the COMMON STAR-FISH, there are five so-called arms, five eye-spots, one at the tip of each arm, and five equivalent elemental components of all the more important viscera. In the SEA-CUCUMBERS, which have elongate worm-like bodies, there is a similar apportionment of the nerves and muscles of the body generally into fives, and also of the branching tentacles which surround the mouth. Tubular locomotive organs, the so-called "tube-feet," are common to all the three types enumerated. The calcareous plates and spinules, while attaining to a maximum development in the urchins, are also abundantly represented in the other groups. In the common star-fish these calcareous elements form within the skin an openly reticulated trellis-like framework, while in the ordinary sea-cucumbers they more usually take the form of innumerable microscopically minute spicules. The two less familiarly known groups of the FEATHER-STARS and BRITTLE-STARS fully agree with the previously enumerated types in their five-fold structural composition. The brittle-stars have almost invariably five arms only, but they are independent outgrowths from the body proper, instead of being prolongations of it, as in the common star-fish.

The FEATHER-STARS, which include some of the rarest and most beautiful representatives of the group, are mostly inhabitants of deep water, and remarkable for the circumstance that either throughout life or in their early phases they are affixed to submarine objects by slender stalks. This peculiarity imparts to the animals such a flower-like aspect that, in conjunction with the indurated calcareous nature of their skeletons, they have received the title of "Stone-lilies." This appellation, however, was originally more particularly applied to their fossilised remains, which occur in remarkable abundance in the older geological strata.

The most familiar British representative of the group is the ROSY FEATHER-STAR, occasionally obtained among seaweed in rock-pools on the southern coast, but more often brought up with the dredge from deeper water. In this form the elongate feather-like arms radiate from the central, relatively small, five-rayed body. There is no supporting foot-stalk in this adult stage, the animal being freely movable, and clinging to seaweeds and other objects by means of a cluster of claw-like filaments developed upon its under-surface. Releasing its hold upon its temporarily selected position, it can crawl about with the aid of the hooked extremities of its arms and their radiating joints. It can also propel itself through the water in a somewhat clumsy fashion by the consecutive flexion and extension of these appendages. This freedom of locomotion was not, however, always possessed by the feather-star. In its early days, and when of very small size, it was affixed to a slender foot-stalk, and dependent for its food on the animalcules and other minute organisms which drifted or swam within reach of its extended arms. The rosy feather-star takes its name from the bright rose-red tints by which it is usually characterised. Individuals of the species are, however, subject to considerable colour-variation. On the Australian coast, where many forms are abundantly represented, examples tinted deep crimson, black, bright golden yellow, or sundry admixtures of these several hues are not uncommonly found associated among a dredge-haul of these elegant sea-stars.

The PERMANENTLY STALKED STONE-LILIES are at the present day of rare occurrence. Up to within comparatively recent years the so-called MEDUSA'S-HEAD LILY was, indeed, regarded as the only living representative of the group. This species has a pentagonal jointed foot-stalk that may be 3 feet long, with five slender appendages developed in whorls at short sub-equal distances throughout its length. From the shallow cup-shaped body at the apex of the stalk a tassel-like bundle of arms is developed, all of these being produced by repeated bifurcation from one of the five equivalent basal stem-joints. Dredging expeditions have within the last quarter of a century revealed the existence of a considerable number of previously unknown species of stone-lilies in the abysses of the ocean, a depth of no less than 3,200 fathoms representing the habitat of one such type.

The Star-fish group is represented by the COMMON FIVE-FINGERS, or CROSS-FISH, as it is sometimes called, and includes a very numerous assemblage of species of varying size and shape and colour. The British seas alone yield some twenty forms. Among the more notable of these is the SUN STAR-FISH, which, departing from the rule of possessing five arms only, has twelve or more, its contour, from which it derives its name, somewhat resembling that of a symbolic sun. The colours of this species are particularly brilliant, consisting usually of a variably patterned admixture of crimson, pink, and white. An extreme contrast in contour to the sun-star is presented by the so-called BIRD'S-FOOT species, in which the body is pentagonal and so flattened out as to somewhat resemble the foot of a duck. In the Cushion-stars the body, while pentagonal, is comparatively thick.

The so-called SNAKE-ARMED SAND-STARS and BRITTLE-STARS constitute a section distinguished from the preceding by the character of the arms, which branch separately from the central body, and are composed of an innumerable series of calcareous joints, which snap asunder under the slightest provocation. The great majority of the species are provided with five simple arms only. In an exceptional form, however, known as the SHETLAND ARGUS, and its allies, these five arms, while simple at their base, bifurcate repeatedly and in geometrical progression to such an extent as to form in life a complex network of writhing, snake-like tendrils, that has been appropriately likened to a Medusa's head. It has been calculated that there are no less than 80,000 terminal arm-subdivisions in adult examples of this species.

Among the Sea-urchin Tribe there are many notable departures from the typical form previously referred to. In some, while the sub-spheroidal form of the case, or test, is still retained, the external spiny armature is greatly varied. In one series these spines are exceedingly long, slender, and of needle-like contour and sharpness. In others, while long, they are abnormally thick and cylindrical, somewhat resembling slate-pencils, for which they are sometimes used as a substitute; or they may be club-shaped, branched, or reduced to flattened plates. In other forms the shell itself is conspicuously modified. With some known as BISCUIT- or CAKE-URCHINS it is flattened out to the resemblance of a cake or biscuit, the spines being minute and inconspicuous. In another group, distinguished as HEART-URCHINS, the shell is oval and bilaterally symmetrical, though the dominant number of five still holds good with regard to the building up of its structural details. One of the most interesting is the LEATHER-URCHIN, so called on account of the flexible and loosely jointed character of its shell, the way being paved by such a form to the normally soft- and flexible-skinned sea-cucumbers. Sea-urchins are to a great extent vegetable-feeders, and the larger species are appreciated as an article of food in many countries, the ovaries, or roe, with which at certain periods the shell is mostly filled, forming the edible portion.

The SEA-CUCUMBERS--better known in the commercial world as Bêche-de-mer, or Trepang--represent the only group which possesses a substantial market-value. Its typical members present an elongate worm-like contour, but progress by means of extensile tube-feet, after the manner of the Urchins and Star-fishes, and have their dental, nervous, and muscular systems fashioned on the same five-sectioned basis. The mouth, which is situated at one extremity of the body, is surrounded by a series of ten or twenty delicately branched or mop-like tentacles, which can be protruded or retracted at the animal's will, and are used for seizing food. The skin of the typical sea-cucumber is more or less soft and flexible, and has embedded within its substance innumerable minute calcareous spinules.

The commercially valuable sea-cucumbers, or bêche-de-mer, are all inhabitants of tropical waters, the North-eastern Australian coast and the Malay seas yielding the most highly prized forms. The Queensland Great Barrier Reef, consisting of a series of coral-reefs extending for upwards of 1,000 miles at a little distance from the Australian mainland, represents one of the most productive areas for this marine delicacy, the bulk of which goes to the Chinese market. The fishery is prosecuted with the assistance mainly of the Queensland natives, who, either by diving or wading on the reefs at low tide, collect the creatures in vast quantities. On being brought to the curing-stations, the animals are emptied from the collecting-sacks into large caldrons, where they are allowed to stew in their own juice for about twenty minutes. Taken out of the caldrons, they are split open and eviscerated, dried for a short interval in the sun, and then placed in tiers on wire gratings in a smoke-house, where they remain for twenty-four hours. They should at this stage have shrunk up to about one quarter of their normally extended size, much resemble charred sausages in aspect, and should rattle like dry walnuts when bagged up for exportation. From £50 to £150 per ton are the prices that the better qualities of bêche-de-mer realise when well cured and delivered at Chinese ports. The chief culinary use to which the cured sea-cucumbers are applied is that of the concoction of soup, the best quality prepared taking rank with that made from swallows' nests. At the hotels and clubs in the leading Australian cities bêche-de-mer soup is held in high favour, and its more extensive introduction on the menu-cards of Western civilisation may be only a question of time.

Many species of sea-cucumbers inhabit British seas, but none possess that density of tissue which is essential for their economic conservation; the majority, moreover, are of comparatively small size, some few inches long only when fully extended, whereas the commercially valuable tropical ones may measure as much as from 2 to 3 feet. The mode of feeding of sea-cucumbers is somewhat interesting; the smaller species, with much-branching tentacles, generally affix themselves by their tube-feet to some object, and, extending their tentacles in all directions, utilise them, like those of a sea-anemone, for seizing any minute and suitable prey which may strike against them. The microscopic organisms on which they chiefly feed abound in the waters they inhabit, and one after the other, the branched tentacles having effected a capture, are gathered together and tucked bodily into the creature's central mouth and apparently half-way down its throat. The larger coral-frequenting species are provided mostly with mop-shaped tentacles. They crawl about leisurely in search of their food, mopping over the ground, and gathering up in their tentacles the minute shells and other organisms on which they subsist, which are collectively thrust with an indrawn tentacle into the throat. In some of the lower forms the tube-feet have disappeared, the integument is thin and semi-transparent, and the worm-like animal crawls about by means of its skin-spinules, which take the form of anchors or grappling-hooks. In an opposite direction they may develop a supplementary covering of dermal plates and a more rigid integument, which indicate their nearer relationship with sea-urchins.

The majority of sea-urchins and star-fishes pass through a series of interesting metamorphoses before arriving at the adult state. The larval phases in these instances are free-swimming organisms, having arm-like processes, strengthened by calcareous rods that have been likened in contour to a clock-stand. A small spherical central area, like a clock in its case, representing the stomach of the larva, develops spicules around it, and becomes the body of the urchin, the other outlying portions becoming gradually absorbed. Some of the brittle-stars and sea-cucumbers bring forth their young in the adult form, nursing them from the egg in special breeding-chambers.

The capacity of a star-fish to renew its lost arms is commonly manifested. A single detached arm, moreover, in such a type as the common five-fingered species, can reproduce its body and the remaining four arms. Fishermen, who are in the habit of tearing up star-fishes and throwing them back into the water, under the impression that they are thus effectually incapacitating them from further injury to their oyster-beds, commit an error, such mutilation tending to the multiplication of their numbers.

In the matter of colour-ornamentation the Star-fish group is richly endowed. Allusion to the brilliant crimson and pink-and-white tints of the British sun star-fish has been already made. As with most animal groups, however, it is amid their tropical representatives that the most striking colour-variations obtain. One form which is common among the coral-reefs on the Queensland coast-line, and much resembles the common British "five-fingers" in size and shape, is brilliant ultramarine-blue. Another large pentagonal species, belonging to the group known as Cushion-stars, has a golden-brown ground, upon which are thickly scattered small bead-like tubercles of turquoise. A third form, not uncommon on the Tasmanian coast-line, which is nearly related to the Bird's-foot species, previously mentioned, is distinguished by tints which range through several shades of crimson to brilliant violet.

Not a few of the star-fishes are notable for their eminent phosphorescent properties. The group of the Snake-armed and Brittle-stars are more especially distinguished in this respect. Many of these species occur in such numbers in comparatively deep water that the dredge may be filled with a tangled mass of their writhing snake-armed bodies. Should it be night when the dredge is brought aboard, and its contents are emptied upon the deck, the spectacle presented as the star-fishes scramble in all directions, their bodies and arms aglow with pale green or blue phosphoric coruscations, is highly remarkable.

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