Part 33

Zoologist, pp. 7054, 7111.

Serpula contortuplicata, a mass of which forms the subject of Plate XXXII., mingled with S. triquetra.

Some in this infant state are seen in Plate XXXIII.

Brit. Annelida, in Rep. Brit. Assoc., 1851.

Zoologist, p. 5976.

Vide supra, p. 135.

Sabella tubularia, represented as occupying the foreground and the left-hand side of Plate XXXIII.

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.

Sabella bombyx, represented in Plate XXXIII., towards the right hand, springing out of a group of Serpula tubes.

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.

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.

PLATE 34.

P. H. GOSSE, del. LEIGHTON, BROS.

ORANGE-SPOTTED SQUIRTER. FOUR-ANGLED SQUIRTER. CURRANT SQUIRTER. ]

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, or a tiny thing which forms a little heap of transparent globules, like pins’ heads, attached to sea-weeds. 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.

The action of the heart in these transparent creatures is equally visible. Below the muslin curtain with its living chambers, down at the very bottom of the body-cavity, there is a transparent sac of membrane, which takes the appearance of a long bag, pointed at each end, but not closed, and strangely twisted on its long axis, so as to make three turns. This is the heart; and within it are seen many colourless globules, floating freely in a clear fluid, which answers to the blood. This circulates throughout the system in the following manner:--We see a spasmodic contraction at one end of the bag, which drives forward the globules contained there; the contraction in an instant passes onward along the three twists of the vessel, the part behind expanding immediately as the movement passes on, and the globules are forcibly expelled through the narrow but open extremity. Meanwhile the free globules surrounding the commencing end have rushed in as soon as that part resumed its usual width, and are in their turn driven forward by the periodic repetition of the pulsation. The fluid, with its globules thus put in motion, is then driven along through the interstices of the various organs of the body, not through a system of closed blood-vessels, some finding their way along the transverse lines that separate the rows of gill-ovals, until they sooner or later arrive at the point where they entered the heart, to take the same course over again.

As in the kindred forms of animal life, the same orifice, the same cilia, the same currents are subservient to breathing and to the reception of food; the stomach digesting the microscopic animalcules which are poured with the entering stream through the receiving siphon. At some distance within the interior of this orifice there are a series of thread-shaped tentacles, affixed in a ring, which we may suppose to exercise some kind of superintendence, by touch or other perception, over the atoms which indiscriminately enter upon the stream, accepting or rejecting. Probably it is in the exercise of the latter discretion that those irregular regurgitations of the current take place, accompanied by a momentary closing of the mouth, that we frequently notice.