Part 18

The protrusion of the fore parts, which takes place in a much more leisurely manner, is performed by quite another set of instruments, formed on a totally different principle. Their action is a pushing or shoving. The instruments for effecting this are the fine but strong bristles which run through the feet. Each bristle is composed of a strong rigid unyielding shaft, having an expanded shoulder drawn out into a point. On one side of this pointed shoulder may be remarked a double row of fine teeth, admirably calculated to catch against any roughnesses of the surface with which they come into contact, against which they then push with the force of the proper muscles. Acting diagonally backwards, from the two sides of the animal, the result of the combination of the forces is that the animal itself is pushed forwards, and so protruded from the mouth of the tube. The feet on the hinder portion of the body are, according to Dr. Williams,[142] modified in structure with express reference to the duties of mopping, sweeping, scraping, and wiping the inferior closed end of the habitation. I think, however, he has forgotten that this end, formed by the animal in its infant state, must now be very much too strait to be reached by any portion of the body, or by any of its organs.

Dwelling in a tubular house, the Serpula would find its breathing organs scarcely available, if these were placed, as in most _Annelida_, in pairs on the body-segments. They are therefore much modified, and that not only in position but in form. They consist of most elegant comb-like filaments, richly coloured, arranged in two rows around the front extremity, one row on each side of the mouth. They are graduated in length, and are so affixed, that, where the rows meet behind, they can be thrown-in, so that a vertical view of the circular coronet shows a great sinus in it. These brilliant gill-tufts form the most attractive feature in these elegant Worms, and are individually most exquisite examples of mechanical contrivance. Examined under a low microscopic power, they present a most charming spectacle. Each filament consists of a pellucid cartilaginous stem, from one side of which springs a double row of secondary filaments like the teeth of a comb. Within both stem and filaments the red blood may be seen, with beautiful distinctness, driven along the artery, and back by the vein (which are placed close side by side) in ceaseless course, constituting a very striking spectacle.

The exterior of these organs is set with strong cilia, so arranged that the water-current is vigorously driven upward along one side of the filament, and downward along the other. Yet the combined result of all the branchial currents is to bring a powerful vortex into the enclosed funnel, the bottom of which terminates in the mouth. The food which sustains nutrition is thus brought to be swallowed, a large quantity of water being at the same time constantly poured into the body; this is discharged (by the agency of a ciliated lining of the hinder parts) in the form of a strong current, which, impinging against the closed end of the tube, is turned upward, carrying with it all extraneous or fœcal matters, and is ever pouring out of the frontal extremity around the neck of the creature. What a beautiful and effective contrivance is this for constantly keeping in a state of the most unsullied cleanness the interior of the house! It reminds one of the fabled Hercules cleansing the Augean stable by driving the river Peneus through it.

ITS STOPPER.

On each side of the mouth there springs a lengthened horny thread, appearing to answer to the antennæ which in other Worms, as well as in Insects and Crustacea, project from the front of the head. Such seems their true relation considered structurally, but in function and office they are very remarkable and quite peculiar. To these organs is assigned the duty of closing up the tube when the animal has withdrawn its gaily-coloured plumes; and for this end, one of these antennæ is much lengthened, and at the end is expanded into a broad trumpet-shaped club, the extremity of which is somewhat concave, and is delicately marked with radiating grooves. This organ is usually painted with the same brilliant colours as the gill-tufts, and by its length, size, and form, makes a very conspicuous feature in the charming Serpula. Its length is such, that when the gill-filaments are rolled up and withdrawn, the conical club enters after all, and is found accurately to fit the trumpet-like orifice of the tube, just as a cork fits tightly into the mouth of a bottle.

Ordinarily those organs which appear in pairs are formed so as to be the counterparts of each other. But here is an exception. One only, sometimes the right, sometimes the left, indifferently, takes the remarkable form that I have been describing, the other being much shorter, and terminating only in a small knob, like the head of a pin. Why should there be this difference? Why this exception to an all but universal rule? The reason is obvious. Yes, obvious enough when seen and noticed; but it tells an eloquent tale of the Divine forethought and care. If both of the antennæ were furnished with the terminal cone, one would interfere with the other in the performance of their closing, corking-up function; they would jam in the doorway, and the tube would be left open. Hence the one is left undeveloped, yet retaining, as I believe, the latent power of expanding into a cone, if it should ever be needed by the accidental loss of the fellow now so furnished. I thus judge, because experience shows me that the conical club is occasionally thrown off, and quickly renewed, in captivity.

My esteemed friend, the Rev. Sir Christopher Lighton, has indeed put on record an example of a Serpula of this species possessing two equal antennæ, that had replaced the single one accidentally lost. They were both perfectly developed, and joined together near the base. Each was decidedly smaller than the single one that had formerly occupied their place.[143] This exceedingly interesting case can, of course, only be regarded as a monstrosity of redundancy, as children are sometimes born with a superfluous finger. But it is valuable as showing that there is a power of development latent in the crownless antenna. I wish very much that the excellent observer had added a note, telling us to what extent the tube was closed, and how the work was performed without mutual interference.

It has been sometimes brought as an objection to our assigning a certain service to certain organs, that the necessity for such service is a gratuitous supposition, since other creatures similarly formed in most respects, and in which we might infer a like need, have no such supply. We may admit the facts, but refuse the reasoning. There can be no manner of doubt that the conical antenna does act as a stopper to the Serpula, as our eyes can see; and surely it would be most unphilosophical to suppose that the function so performed is not serviceable to the creature. Yet its near cousins, the Sabellæ, similarly constructed, and of similar habits of life, and as we should have _à priori_ supposed, quite as liable to injury in the same direction, are entirely destitute of this contrivance for protection, and of anything compensating for it, so far as is known. Why the need of one should be met by such a beautiful contrivance, while the same need in the other is wholly unmet, though both are formed by the same Infinite God, is one of those unanswerable questions which, while they leave unimpeached His wisdom, make us deeply conscious of our own ignorance.

We find numerous examples of this genus _Sabella_ in our confused heap of tangled life and death. By their vigour and their abundance we have proof enough that their wants _are_ supplied, though they do not enjoy this special contrivance: they manage to live and thrive and enjoy themselves, with open doors, taking all risks of insidious robbers,--such, for instance, as that vile burglar, the Longworm,[144] that we found under a stone the other day; which is ever on the watch to insert its snaky head within the unprotected tube, and to tear away with merciless clutch the beauteous gill-tufts.

One species of this genus[145] can by the cursory observer be distinguished from the Serpulæ, only by this absence of the antennal stopper. For it dwells in a shelly tube, essentially resembling those which we have just been examining. It has peculiarities of detail, however. It is never found associated with numbers of its fellows in agglutinated groups, but always, so far as my experience goes, singly. It is more common on shells than on stones, generally attached to the old valve of some cockle or scallop. It is straight or nearly so, never at least contorted. Attached only for a very short portion of its smaller end, perhaps for an inch or so, for which it creeps along the surface, it then rises into a more or less erect position, extending sometimes to a height of seven or eight inches perfectly free. The tube is of about the same diameter as that of the Serpula, but is slighter in structure, or perhaps it appears so, because it is destitute of those expansions which here and there in that species indicate the trumpet-lips of successive stages of development. The extremity of the tube here is simple, not expanding. Slight annular rings, however, do here and there vary the shelly surface of the tube.

The gill-tufts are ample; they are two, considerably infolded, consisting of about forty-five filaments each, which are much longer and slenderer than those of the Serpula, the last filaments of the volutions diminishing rapidly. The secondary filaments, or pinnæ, are very fine and very numerous, so set on the main stem that the two rows form the sides of a narrow groove, facing inwards. The whole is yellowish-white with eight or ten bright scarlet dots set with intervals all along the back or outer side of the stem. When fully protruded, the base of the gills, and even a good deal of the neck, lolls out of the tube. If the animal be removed, the body is seen to be white, elegantly banded with scarlet, and furnished with a broad translucent collar, edged with scarlet: this collar ordinarily lines the mouth of the tube.

From the length and isolation of its shelly tube this is a remarkable species: the great tenuity of its filaments, however, requires a lens to bring out their beauties; but with this aid, the arrangement of the rich scarlet bands and spots on the pale yellow ground cannot fail to evoke admiration.

SABELLÆ.

In general, the Sabellæ inhabit tubes which are not calcareous or shelly; they are composed of a soft flexible substance somewhat resembling wet parchment, made of a secretion from the animal’s body, in which the impalpable muddy sediment which the waves agitate, consisting of decomposed organic matter for the most part, is interwoven. The tissue so made is sufficiently tough and enduring, retaining its form long after the animal has died out of it.

In our dredge-hauls we find a pretty little kind[146] common enough, which lives in association, the tubes apparently from half-an-inch to an inch in length, forming dense masses on stones and shells, and projecting in every direction. A dozen or more may be in one group, and when all are alive, one or another protruding or retiring every moment, it makes a pretty object.

The gill-filaments are nine to eleven in each row, of a yellowish white, occasionally patched with dead-white, or red-brown: delicately and densely pinnate. The filaments, in the act of protruding, are closed together like a straight bundle of rods which suddenly fall open at the ends. In this moment of unfolding, their tips are seen to be a little hooked inwards. The tube is about as large as a crow-quill; under a lens it appears speckled, as if the inorganic matter imbedded in it were grains of the finest sand. On carefully removing all the surrounding tubes and other objects so as to isolate one, we see that it is truly about three inches in length, but that two-thirds of the whole are prostrate and adherent; this basal portion is horny and pellucid, no mud entering into its texture. The animal when extracted is an inch in length, of which the gill-tufts form one-third.

Mingled with these there are one or two specimens of a much more imposing species, the Hook-plumed Sabella.[147] It grows to a large size, the crown of gill-filaments sometimes attaining a height of an inch, and the same diameter. The two rows are incurved in regular spirals of half a turn, each consisting of about eighteen filaments, which are rather stout, the whole crown sometimes taking the form of a funnel, sometimes that of a cup, often arching inward at the tip. Their pinnæ are long and close, the two rows forming a groove, but nearly parallel. Each primary stem is set along the back with twelve pairs of feather-like processes, hooked downwards;--a very remarkable character, and one by which this species may be in a moment distinguished. Their colour is pale red-brown, mottled irregularly with deep brownish purple and with white; there is a pair of brown specks at the origin of each pair of hooks. The base of the crown is always concealed in the mouth of the tube, but it springs from a narrow frilled membrane of pure white. The body is destitute of a thoracic shield, or conspicuous collar. The tube is largely composed of soft homogeneous mud, usually of a pale purplish hue, of about the thickness of the shelly tube of _S. tubularia_.

TUBE-BUILDING.

The process of building the mud tubes of the Sabellæ is a very interesting one. It is performed, according to my own observations,[148] mainly by means of the gill-filaments and their pinnal grooves. The filaments are bent-over, till the inner or grooved face comes in contact with the soft mud on which the animal is lying, when the sensitive pinnæ close on a minute portion of the mud, taking it up in a pellet, which is then fashioned by the form of the groove; the filament is now erected, and the pellet, passing down the groove to the bottom by means of the cilia, is delivered to the care of two delicate moveable organs, like leaves or flaps, which place it on the edge of the tube, and then shape and mould it, smoothing both surfaces. Doubtless, either from these organs, or from some other part of the circumjacent region, the glutinous secretion is at the same time poured out, which consolidates the mud, and forms the true basis of the tube.

FOOTNOTES:

[133] _Antennularia antennina_, figured in Plate XXXI., springing from the lower left corner. It will be easily recognised from the description.

[134] For details and figures of these developments, I beg to refer the reader to my _Naturalist’s Rambles on the Devonshire Coast_.

[135] _Scalpellum vulgare_, seen in Plate XXXI., in the position described in the text.

[136] _Balanus balanoides_, of which a group is seen in the extreme left of the foreground in Plate XXXI.; _B. porcatus_, a single specimen, is a little to the right.

[137] _Lepas anatifera_. A group, the size of life, is seen depending in the upper right-hand corner of Plate XXXI.

[138] _Pyrgoma Anglicum_, of which three specimens are seen attached to a _Caryophyllia Smithii_, at the left side of Plate XXXI.

[139] _Zoologist_, pp. 7054, 7111.

[140] _Serpula contortuplicata_, a mass of which forms the subject of Plate XXXII., mingled with _S. triquetra_.

[141] Some in this infant state are seen in Plate XXXIII.

[142] Brit. Annelida, in _Rep. Brit. Assoc._, 1851.

[143] _Zoologist_, p. 5976.

[144] _Vide supra_, p. 135.

[145] _Sabella tubularia_, represented as occupying the foreground and the left-hand side of Plate XXXIII.

[146] I have figured a group in the centre of Plate XXXIII. I cannot satisfactorily identify it with any species described in Grube’s _Fam. der Anneliden_. It has some affinities with the _Sabella penicillus_ of Müller; and still more with _S. gracilis_ of Grube. This latter is defined, however, as wanting the two naked threads by the mouth, which in my little species are sufficiently conspicuous. I must leave it undetermined.

[147] _Sabella bombyx_, represented in Plate XXXIII., towards the right hand, springing out of a group of Serpula tubes.

[148] _Intellectual Observer_, vol. iii. p. 77.

XII.

DECEMBER.

December is here, with its short days, its feeble watery sunshine, its frequent gloom and mist, its hanging leaden skies; in short, as the poet describes it,--

“Sullen and sad, with all his rising train; Vapours and clouds and storms.”

It requires some zeal in the pursuit of scientific lore to leave the glowing fire and the pleasant book, the luxurious arm-chair and the elastic carpet, and to venture down to the wild sea-beach, to poke and peer among the desolate rocks. Yet even now we may find a few bright days, when Nature abroad looks inviting, and when an hour’s marine research will prove neither unpleasant nor unsuccessful.

SQUIRTERS.

On such a noon, then, calm and quiet, the sun bright and cheerful, if low and feeble, the tide tolerably low and the rocks accessible, we hie down to some one or other of those ledges which have so often already yielded their treasures to our search, and begin our wonted labours at turning over the heavy angular masses. We soon find, attached to the under surfaces of these, what seem to be irregular blobs of coloured jelly of somewhat firm consistence, as if an invalid had been here eating his calves’-foot jelly, whose trembling hand had dropped sundry spoonfuls on the stones. Some appear as flattish shapeless drops, but others take more elevated forms, like sacks set on end, and usually displaying two mouths. One of these is of a pellucid yellowish green, or olive hue, with a cloudy spot of rich orange in the interior. A slight shrinking from the touch, a yet closer contraction of the projecting points, is the only token of life that we can discern in it now; but if we place it in an aquarium,--not forcibly removing it from its attachment, but lifting the shell or stone on which it rests; or, if this be too large, detaching the fragment with a chisel,--and allow it to remain a few hours undisturbed, we shall see evidences of a vitality, indubitable if not very active.

The whole creature is now much plumper and more pellucid; it stands up boldly from its base on the stone; its upper portion is much lengthened, and the two wart-like eminences have become two short tubes with gaping extremities, appearing as if they had been soldered together side by side, of which the one is considerably higher than the other.[149]

We have before us one of the _Tunicata_, an order of molluscous animals which are closely allied to the _Conchifera_ or bivalves, but somewhat lower in the scale than they. It has no shell; that is to say, lime is not deposited in the outer investment, so as to give it the hard, rigid, solid texture of shell; but the internal organs, which are essentially similar to those of an Oyster or a Sand-gaper, are enclosed in a tough leathery coat, known as the _test_, which is in fact a closed shell destitute of lime. The eminent physiologist, John Hunter, who had dissected some of these homely Squirters, as they are familiarly called, recognised, with his wonted acumen, the structural similarity of their leathery envelopes to the stony shells of the lower bivalves; and, associating them in a group, called them “soft-shells.” The naturalness of this group, since called _Tunicata_ by Lamarck, has been recognised by modern zoologists.

THE GILL-SAC.

If we watch our Ascidia for a few minutes, we perceive that at irregular intervals one or both of the gaping orifices are suddenly closed and contracted, commonly both at the same instant. They are, however, soon opened again; and we may discern, especially if the specimen is in a glass vessel, and we watch it by the aid of a lens, with the light of a window at its back, that a current of the surrounding water flows from all sides to the taller orifice, and pours down its tube; while occasionally we see the ejection of a stream from the orifice of the shorter tube. Thus we have here a receiving and a discharging tube, the exact representatives of the two siphons in such bivalves as _Pholas_, _Venus_, etc. The former leads down into a capacious sac in the interior, the walls of which constitute the breathing apparatus. The inner surface is marked by regular parallel ridges which run in a horizontal direction; and these are again connected by vertical ridges at right angles, very numerous, enclosing a vast number of oval compartments. The sides of these are richly ciliated; and if the whole apparatus be carefully dissected out, and laid upon the stage of the microscope, the course of the ciliary currents may be distinctly seen, continuing with unabated vigour and with unfaltering precision for a long time after the severance of the organ from the body of the animal. But all this is seen to most advantage, if we select one of the smaller species, which are brilliantly transparent, such as one which grows in groups of elegant tall vases, about an inch in height, around the edges of our rocky pools,[150] or a tiny thing which forms a little heap of transparent globules, like pins’ heads, attached to sea-weeds.[151] In either of these, placed in a stage-trough of sea-water, we can watch at leisure the performance of the various vital functions in healthy action, with the knowledge that the little subject has not been martyred to science, but is all the while enjoying its humble life with perhaps as much zest as if it were still environed by the rough walls of its little native basin of rock.

In the tiny pin-head of clear jelly, the microscope displays the branchial sac hanging free in the cavity, like a bag of clear muslin. The oval cavities divided-off by the rectangular ridges are about forty in number, around each of which the ciliary waves incessantly roll, as running spots of black. It is a very charming spectacle to see so many oblong figures set symmetrically, all furnished on their inner surface with what look like the cogs or teeth of a mill-wheel, dark and distinct, running round and round with an even, moderately rapid, ceaseless course. These black, well-defined, tooth-like specks are merely an optical effect; they do not represent any actual objects, but only the waves which the cilia make: the cilia themselves being hairs, so fine as to be defined only with high powers. Occasionally we see one or other of the ovals suddenly cease its movement, while the rest go on; and now and then the whole are arrested simultaneously, and presently all start off again together, with a very pleasing effect, as if we were looking at the wheels of a very perfect and complex piece of machinery. These phenomena appear to indicate that the movements are under the control of the animal’s will, capable of being suspended or continued, wholly, or in any degree, at pleasure; which is not the case in the higher animals; our own respiratory movement, for example, as well as the pulsations of the heart, going on without the concurrence of our will, and even without our consciousness.

THE HEART.