Confronted by danger, the animal closes its front door by retracting until the disc presses immovably against the circumference of the tube, the retraction being so sudden that a frail spurt betrays the whereabouts of an otherwise secret dwelling-place. In the centre of the disc is the first segment, from which the frontal fringe is extended in the form of an array of keen bristles as a defensive weapon. With the lid at one end and the armed disc at the other the animal enjoys security and comfort, and when unsuspicious the "shoulders" protrude, the head meekly following. The tentacles are serrate and glitter like tinsel, possibly for the fascination of the minute forms of life which the tube-dweller consumes. To enable it to retract and emerge quickly the animal is provided with a series of tufts of bristles on the back and on the ventral surface of the body with a row of toothed "pads," which fulfil the dual office of grapplers and feet.
With what skill and patience does this pectinarian construct its ornate habitation! How artfully does it pick and choose among the tiny shells and grit! With what rare discretion rejects the unfit, and with what satisfaction retains a neat and dainty item of building material! How deftly does it arrange its courses and bonds, cementing each fragment in its place until a perfect cylinder, proportionate in dimensions, uniformly expanding in circumference, smooth within, rugged without, scientifically correct in design, is the result! How apt, too, the frictionless lining! And all this laborious neatness and precision absorbed in the construction of a tenement which has no time! Does the inmate possess any sense of duration? Addison (quoting a French authority) says that it is possible some creatures may think half an hour as long as we do a thousand years! The magnificent mind of the modern biologist regards a million of years as a mere fag-end of time. The industrious worm which has built so choice a home may have enjoyed the sense of comfort and security for a period representing an honourable age, while, according to the standards of man, the home was not worth the building for so short a tenancy.
Do we not see in this astonishing example a highly successful effort of a marine worm to improve on the condition and habits of its barbarous ancestors? Analyse a bulk sample of the building material, and you shall find it not dissimilar from the shell of a mollusc, and the interior film—no doubt a secretion of the animal—is to be safely accepted as analogous to the silky smoothness which molluscs (often of rough and rugged exterior) obtain by nacreous deposit and which finds its culmination in the goldlip mother-of-pearl?
Still higher in the series, so far as the construction of a tenement is concerned, is that known as the SERPULA, a worm which constructs a calcareous tube more or less loosely convoluted and adherent to a shell or stone or coral, or sometimes entwined into a self-supporting colony.
Another worm builds of sand or mud, with a rough casting of fine gravel and shell-grit, a habitation similar in design to that of the serpula, though on a less complete and authoritative model; indeed, it would almost seem that the latter had designed its tenement after the fashion of that of its poor relation—that the one made a study in mud which the other reproduced in carbonate of lime. But the most curious fact is that a true mollusc (VERMETUS) so far departs from the fashion prevalent in the molluscan world of building a spiral shell, that after beginning one in proper spiral mode it elongates itself in vermiform manner and forms an irregular serpuloid tube on the surface of larger shells or stones just as the SERPULA does; so that without examination of the animal one may easily be mistaken for the other.
What a contrast is here—on the one hand a lowly worm learning to build a solid if rude shelly covering for its tender body, on the other a relative of the elegant, many-whorled TURRITELLA forgetting its high station and degenerating to the likeness of a worm. No doubt it is really a case of degeneration from the acquirement of fixed habits, just as when a lively young crustacean larva gives up its free independent life and glues its head to a stone—what happens? Why, he becomes a mere barnacle instead of a spritely shrimp as he might have been! Let mankind take note and beware.
Another group of worm-like or snaky creatures common on a coral-reef are the sea-cucumbers or bêche-de-mer. In my experience the most singular branch of the family is at once the longest and thinnest, for it resembles a snake so closely that at first sight the observer subconsciously assumes an attitude of hostility. There seem to be two varieties of the species. One is much more ruddy in appearance than the other, and its body is the smoother; but they are much alike in physique and helplessness. The figure of a sausage-skin four or five and even six feet long, and capable of elongation to almost double, containing muddy water in circulation and one end exhibiting a set of ever-waving tentacles, conveys a not unflattering notion of the animal as it lies coiled among the coral, half hidden with algae. Far too feeble to be offensive, it suffers collapse on alarm—that is to say, if such a violent mental and physical ill can befall an animal of such crude organism. At least, the tentacles are withdrawn, nor will they be protruded until some sense—unlikely to be either sight, hearing, taste, or touch, but probably nervous tension acutely susceptible to vibrations—tells that danger is past. Then the tentacles are shyly exhibited and the agitations of the animal are renewed.
Throughout the length of the body of the more remarkable of the two species of which I may speak on first-hand knowledge are four rows of bosses, closely spaced, which when the animal has dragged its slow length along to the utmost limit diminish into mere wrinkles, and disappear altogether when it is slung across a stick, and the fluid contents, being precipitated, congest and woefully weight each end, sometimes to the bursting-point. The bosses of repose seem to indicate so much length in reserve. A dozen simple tentacles, sword-shaped, with frayed edges, and about an inch and a half long, indicate the head without decorating it, for they are of an inconspicuous neutral tint, closely resembling the alga among which the animal slowly winds its way.
The progress of all species of bêche-de-mer is sedate and cautious, and this, probably the longest and the weakest and limpest of all, surpasses the race in deliberateness. It cannot move as a whole, so it progresses in sections. When the head has been advanced to its utmost, about the middle of the body an independent series of succeedant ripples or wrinkles manifests itself and travels consistently ahead, while farther towards the rear another series follows, and so on, until the lagging tail is enabled to wrinkle itself along. But the animal is endowed with the capacity of quite suddenly retracting its forepart like the bellows of a concertina, and when so compressed to heave it backward or in any direction, so that an immediate change of route is possible. The retraction and uplifting of the foreshortened part is astonishingly rapid in view of the methodic movements of the animal as a whole. It is also notable that when the retraction takes place the tentacles are entirely withdrawn, otherwise they are for ever anxiously exploring every inch of the toilsome way. Scientific men have entitled one of the species—possibly the very one blunderingly introduced—SYNAPTA BESELLII, and brief reference is made to it elsewhere.
One member of the great "sea-cucumber," or BÊCHE-DE-MER, family is especially noticeable because it is decorated with colours of which a gaily plumaged bird might be envious, though it has no other claim to comeliness. Most primitive in form—merely a flattened sac, oval and four inches long by three inches broad, with a purple and white mouth puckered as if contracted by a drawn string. Its general tint is grey; longitudinal bands of scarlet, green, violet, and purple radiate from the posterior and converge at the mouth, the hues blending rainbow-like. The brighter colours seem to have been carelessly and profusely applied, for they run when touched and smear the fingers. Among a family generally sad-hued and shrinking so conspicuous an example is quite prodigal and invites one to ponder upon the sportfulness of Nature. What special office in her processes does this fop of the species with prismatic complexion perform?