ECHINODERMA.
Six table-cases contain the dried Echinoderms arranged in systematic order. The seventh is devoted to preparations, models, and figures illustrative of the structure and life history of various members of the group.
An inspection of that Case and the accompanying woodcuts will make clear the distinctive characters of the Echinoderma. Unlike that of a Crayfish or a Mussel, the body does not appear to be divided into two equal or symmetrical halves, though it really is; this is due to the possession of a number of rays, of which there are ordinarily five. The skin is strengthened by the deposition in it of carbonate of lime, which may be in the form of continuous plates or bars, or of separate scattered spicules. A series of tube-feet or suckers (podia) are generally developed along each ray, and these are supplied by a system of water-vessels peculiar to Starfish and their allies. These rays are often called “ambulacra.”
Fig. 1.
A. Anchor and plate of Synapta. B, C. Tables of Holothuria impatiens; and D. Holothuria atra: from various aspects. E. Spicule from sucker of Stichopus variegatus, magnified about 200 times.
Fig. 2.
Diagram of Water-vessels.
c.c. Circular canal, with p.v., its Polian vesicles; from it a radial canal (v.c.) is given off along the lower surface of each arm; this supplies, by side branches, the suckers, s; connected with each sucker is a contractile swelling or ampulla (a). The circular canal is in connection with the exterior by s.c., the stone-canal, and opens to it by the madreporite (m).
Fig. 3.
Figure of a Starfish (Asterias rubens).
In the ray marked I. the skin has been removed from the upper surface, and the ambulacral ossicles (ao) and the podia (s) are seen in situ; the blind outgrowths (c) from the central stomach (sp) have been dissected out. In II. the gonads (g) are exposed; and in the centre above the stomach the rectal glands (rg) are to be seen. The anus (a) is seen to be subcentral in position.
In the body of the Starfish (Fig. 3) the arms are seen to be continuous with the disk and to contain portions or prolongations of the chief organs. The middle of the arm is occupied by two rows of hard pieces (ambulacral ossicles), the fellows of which make an open angle with each other, and so form an open ambulacral groove; along this we find the suckers, the water-canal that supplies them, the blood-vessel of the arm, and a nerve-cord. At the centre of the disk is the mouth. The ossicles at the sides of the arms bear spines, which vary in different species; the surface of the back is supported by a network of hard pieces, and through the intervening spaces there project membranous pouches, which are respiratory in function. The modified plate on the upper surface opens into a tube by means of which the water-vessels communicate with the exterior; this plate is known as the madreporite (Fig. 2, m).
The organs for masticating the food are most highly developed in the regular Echinoids, where the complex apparatus known as the “Lantern of Aristotle” is found (Case 38) to consist of five sets of pieces; the tooth is strong and bevelled at its free end; it is supported by triangular jaws on either side, a pair uniting and having the form of an inverted pyramid; these alveoli are connected with their neighbours by oblong pieces (falces); above these there are elongated bars, which are hinged on to the inner end of the falces and have their outer ends free. The whole lantern is connected to the test by muscles which pass from its sides to the auricles or upstanding pillars which lie round the mouth; and, owing to this muscular apparatus, the teeth are capable of complicated and various movements.
In the Ophiuroids the edges of the mouth-slits are provided with short spinous processes, varying a good deal in arrangement, but never having, apparently, any other function than that of a filtering-apparatus; in the Starfishes the plates round the mouth have a supporting function only; in Crinoids and Holothurians the mouth is unarmed; the latter are often remarkable for a deposit of calcareous plates in the walls of the gullet, and in the former the grooves on the arms are the lines along which food comes to the mouth.
Echinoids live on seaweeds and the animals that are found on them; such as have no teeth, like Spatangus (Case 32), use their spout-like mouth to take up the sand and débris on which they move, and from which they extract some nutriment. Ophiuroids live on the smaller foraminifera; Asteroids on dead fishes (as line-fishermen well know), oysters, and other molluscs, and even on specimens of their own particular species; Holothurians on shell or coral débris and the minute organisms it contains; and Crinoids on small tests of foraminifera and on the adults of small and larvæ of larger crustacea.
In a number of Echinoids and Asteroids some of the spines are specially modified to act as seizing-organs—the free end being divided into two, three, or rarely four pieces, which are moved on one another by special muscles. These minute organs were regarded by earlier observers as parasites, and were named pedicellariæ; they may be movable, when they have a stalk, or the stalk may be absent and the valves sessile. Considerable difficulty attaches to the determination of the use that these organs may be to their possessors; but there is reason to suppose that they may act as cleansing-organs by removing minute particles of dirt, and as temporary organs of fixation, while M. Prouho has observed their use as organs of defence.
Echinoderms move but little; the unstalked Crinoids, if they cannot find stones or worm-tubes around which to attach themselves, swim by beating the water with their delicate arms, five being raised and five depressed alternately. The Echinoid or Asteroid is able to move by the aid of its podia or so-called ambulacral feet, which become erected by being filled with water, and are then contracted; by means of this contraction movement is effected; a similar kind of locomotion obtains with the pedate Holothurians; in the Ophiuroids the flexible arms either serve as the organs of movement, or act as an apparatus whereby the creature becomes coiled round the branches of corals (see Case 20).
Echinoderms are often of exceedingly bright colours, as is shown by the pictures on the wall, and are very conspicuous objects; this may, apparently, be associated with disagreeable tastes or odours; sometimes they cover themselves over with seaweed, and so hide their brilliancy; the spines of some forms are exceedingly painful to the touch, and the stout plates of some of the Goniasters must form admirable organs of protection. The power of restoring lost or injured parts is one of the most remarkable points in the Echinoderm organization (see Case 6).
Echinoderms are of great geological age, and were very abundant in earlier periods of the world’s history. Two groups (the Blastoids and Cystids) have completely disappeared, and the Stalked Crinoids (Lily-Encrinites) are far less common than they used to be. Echinoderms are now found in all seas, and extend to great depths of ocean; many of the species have exceedingly wide areas of distribution, and most are characterized by their gregarious habits, a large number of specimens of a single species being generally obtained by the dredge. They are most abundant in the tropical seas.
Most Echinoderms lay their eggs in the water, where the larvæ are developed and swim about freely; but in a few (Hemiaster, Ophiacantha vivipara, and others) the young do not pass through any metamorphosis, for the eggs are placed in special pouches of the body of the parent, in which they are hatched. The free-swimming larvæ of the other Echinoderms pass through a series of remarkable changes (Figs. 4 and 5); these are illustrated by the twelve models of various forms of larvæ exhibited in Case 36; in Case 35 is a set of models showing in detail the changes undergone by a single species (Asterina gibbosa). A portion only of the body of the larva is converted into the substance of the perfect animal; the rest is either absorbed by the growing animal, or shrivels up and disappears.
Below the twelve models in Case 36 may be seen a representation of three stages in the history of the Feather-star (Antedon bifida). The larvæ of this Echinoderm are not free, but are attached by a stalk (Fig. 6); in the common Feather-star and other Comatulidæ the stalk is found during larval stages only; in others, such as Pentacrinus, it persists throughout life.
The presence or absence of this stalk has been taken as the first character of importance in the classification of Echinoderma which may be divided into two groups:—
A. Pelmatozoa,[[24]] or Echinoderms provided with a stalk throughout life or in the larval stages only. To this group belong the Crinoidea, and the extinct Blastoidea, and Cystidea.
Fig. 6.
Pentacrinoid stage of Antedon rosacea.
a, arms; b, basals; r, radials; s, stalk.
B. Echinozoa, or Echinoderms without stalks at any time of their existence. To this group belong the Asteroidea, Ophiuroidea, Echinoidea, and Holothurioidea.
Crinoidea.—This Order may be described as stalked, globular, or cup-shaped Echinoderms, in which the oral surface of the calyx or disk looks upwards, and in which five jointed and generally branched rays arise from the central disk. Their joints have jointed pinnules at their sides, and the sucking-feet have the form of tentacles.
The stalked representatives of this Order are placed on tables and brackets near the south door, and are worthy of being particularly noticed for their fine preservation, size, and beauty. The largest specimen of Pentacrinus decorus was taken on a telegraph-wire, to the covering of which the stalk of the Crinoid is still attached. Metacrinus is a more lately discovered genus, which appears to be confined to the eastern seas.
A few dried unstalked Crinoids are shown in Table-case 1; these show the leading modifications of structure in the two great genera Antedon and Actinometra.
Fig. 7.
Comet form of Linckia.
Asteroidea.—This Order comprises Echinoderms with a depressed body of pentagonal or star-like shape, to the ventral surface of which the ambulacral feet are confined. The rays are more or less elongate movable arms, with skeletal structures, which consist of transversely arranged, paired, calcareous plates, articulated with each other like vertebræ, the series extending from the mouth to the end of the arms. The groove in which the ambulacral feet are arranged is uncovered.
Typical specimens of this Order are exhibited in Cases 2 & 3, in which the great variety of form in the genus Asterias and beautiful examples of Acanthaster are shown. Cases 6 & 7 contain specimens illustrating the curious habit of self-mutilation possessed by so many Echinoderms; among Starfishes, and notably in the genus Linckia, the single arms separated from the disk are able to develop a fresh disk and arms, and so to multiply the species. Cases 9–11 contain fine series of Oreaster.
Ophiuroidea, or “Brittle-stars.”—These Echinoderms appear to resemble the ordinary Starfish[[25]]; but they differ in having the organs of digestion, respiration, and reproduction confined to the disk, the arms having merely the function of locomotor organs. The arms therefore are more slender and cylindrical in form, and are sharply distinct from the disk; the separate joints consist of two central ossicles, which leave only a narrow canal between them, and these are covered above, below, and at the sides by specially developed investing plates; the lateral plates bear spines, which are always comparatively short and delicate, as compared with the spines found at the sides of the arm in starfishes.
The principal types of this Order are exhibited in Cases 17–22; the most exquisite of them are the forms whose arms are divided and subdivided till they end at last in the finest threads, as in Astrophyton, the so-called Basket-fish or Gorgon’s heads.
Echinoidea, or “Sea-Urchins,” are Echinoderms in which the rays are not free, as in the Starfishes or Brittle-stars, but unite to form a compact, spherical, heart- or disk-shaped test; this test is covered with spines, which may attain to a great length, as is shown in the fine example of Diadema saxatile from the Andaman Islands; some of the tests are flexible and very fragile. Owing to the quantity of specimens that are sometimes dredged at one spot, the naturalist has been able to gain a better idea of the range of variation in the species of Echinoderms than in some other divisions of the Animal Kingdom; an instructive series, showing the variations of Echinometra lucunter, is shown in Case 28.
The genus Hemiaster offers an example of an Echinoderm in which the eggs are laid in special pouches; the hinder ambulacra are deepened to form pits, which are guarded by specially elongated spines (see Case 34); in these pits the young pass through all the stages of their development.
The minute structure of the spines of Sea-Urchins is illustrated by a series of figures on the wall.
The Holothurioidea, or Sea-Cucumbers, form the last order of Echinoderms. Their body, as indicated by their English name, is elongate, subcylindrical, with a more or less flexible integument, according to the extent of the reduction of the calcareous skeleton; the mouth is at one end of the body and surrounded by tentacles, the vent at the opposite end.
As these animals cannot be shown in a dried state, some of them, preserved in spirit, are placed in Wall-Case IV. According as they have or have not the sucking-feet of the Echinoderma, they are ordinarily divided into the Pedata and the Apoda; the latter are represented by Synapta, which may attain to a great length, and by Chiridota; the Pedata are illustrated by the genera Cucumaria, Psolus, and Holothuria. Deep-sea investigations have revealed the existence of another group of specially modified Holothurians—the Elasipoda; these are remarkable for their well-marked bilateral symmetry and the distinctness between the dorsal and ventral portions of the body; the prominent processes on the dorsal surface are not contractile.
An exhibition of some interest is to be found in a Table-Case against the wall, in which there are various specimens of the edible Holothurians—trepang or bêche-de-mer; these were all bought in the market at Canton, and may be taken to be typical of the kinds offered for sale in various eastern countries.