Part 19

REPRODUCTION OF ORGANS.

I find among my notes a record of a specimen of Phyllodoce, which, if not identical with this, was closely allied to it, in which I observed the continuance of vitality after the severance of the creature into parts. It was sent to me from Torquay, when I lived near London. When it arrived I found that about an inch and a quarter of the anterior extremity was detached from the remainder, which measured about four inches. The former was motionless, contracted, and seemed lifeless; the latter moved freely. I put both into an old aquarium. The long posterior portion glided about among the stones for two days, exactly like a living healthy animal; the anterior part remained motionless and contracted until the third day, when I saw it also gliding over the stones in a most lively manner, rearing its head, and feeling about in the manner of a caterpillar. Eight days after its arrival, the head portion was still active and apparently healthy, but the hinder part had become motionless and was evidently dead. I find no further record of the case, and probably the anterior part ultimately died without reproduction; but the length of the period of its survival in apparent vigour, renders it not improbable that in the open sea, under the influence of abundant oxygen, and suitable food, the wanting parts might have been renewed to the fore-part, if not to both.

To return to my more recent captive, however. I killed it for cabinet preservation by putting it into fresh water, where it presently died, with the noticeable circumstance that it threw out mucus in such profusion that the whole body was enveloped in a mass, much thicker than itself, of clear jelly, excessively tough and tenacious.

The most common members of the class Annelida, that we meet with in these situations, are different species of the genus Nereis, which are for the most part worms of considerable size, usually brown or green, with a changeable metallic lustre above, and brilliantly pearly beneath. They have a distinct head, of a squarish form, terminating in two swollen fleshy knob-like antennæ, and furnished with four pairs of thread-like tentacular cirri, which project on each side like a cat’s whiskers. The body is plump, though somewhat flattened, and bears on each side a row of fleshy foot-warts, which are pierced for the extrusion of the curious bristles that are so characteristic of these marine worms.

PEARLY NEREIS.

We can scarcely turn one of these flat stones which lie half-buried in sandy mud at the water’s edge without finding one or more of this tribe. Let us try. Here at once is a specimen, one of the finest as well as one of the commonest of all. The upper surface is of a warm fawn-brown; but the beautiful flashes of iridescent blue that play on it in the changing light, and the exquisite pearly opalescence of the delicate pink beneath, are so conspicuous as to have secured it the title of “Pearly,” par éminence. As you gaze upon it you see the great dorsal blood-vessel or heart, as a dark red line running along the middle of the back. This, at irregular intervals of five to fifteen seconds, contracts almost to invisibility, and then expands again, taking the form of a string of beads in each process.

This, however, is only a part of the great circulating system in this fine worm. In it the red blood flows from the tail towards the head; at the under side of the body, there is a similar vessel (the ventral), through which the blood flows from the head towards the tail. Besides these, there are four other longitudinal vessels in the interior of the body, one above and one below the intestine, and one on each side, all of which are connected by transverse branches. Thus there is a double circulation. “The great dorsal,” says Dr. Williams, “the reservoir of the centripetal streams of the body, may be likened to a right ventricle (the lungs cut off), and the great ventral to a left ventricle. The duty of the former is to collect the refluent blood of the system; of the latter to circulate it again.”

Our Nereis is furnished with two pairs of dark blue eyes, and with an evertile proboscis, which consists of two segments. The one first protruded is beset on its upper side with some largish horny points, arranged in two groups, which are continued round to the under side in a band of irregular lines of points. The second segment has four groups of points, the groups of oblong outline, radiating from the strong black jaws, which are protruded last, and can be widely opened and closed.

Here, too, we have an Annelid of very different form and appearance. It is a mean-looking worm, about an inch and a half long, of flattened shape, blunt at each end, apparently covered with a smooth skin of a dull brown colour; but on being touched it presently throws itself into elegant serpentine curves, and then what appears to be the upper skin is seen to be composed of a great number of round flat membranous plates or shields, arranged in two rows overlapping each other. These, though of large size, are attached to the body only by a small point in the centre of their under side, so that when the animal moves, the edges of these shields are lifted, and reveal their true structure, sliding upon one another in a singular manner.

In captivity I have found the animal inert, prone to seek concealment in the corners of the glass vessel beneath other animals or stones, and remaining still for a long time together; but, if disturbed, manifesting impatience and much agility, swimming through the water with strong lateral undulation, as already described. I have been struck with the deciduous character of the broad leaf-like shields. As soon as the animal was in my possession, one and another of these appendages was cast off at almost every time that I roused it to action; and even when allowed to remain quiet, the denuding process went on, so that in a few days it was nearly half naked. This process of self-mutilation will sometimes go on till scarcely a single shield remains out of the double series.

SCALE-WORM.

This dull-coloured Scale-worm presents peculiarities of structure which are eminently worthy of careful examination. The body, like that of the other species we have just been considering, is composed of many rings or segments, each of which bears on its upper surface a little wart or tubercle on each side, to the summit of which is slightly attached the circular leaf-like shield just described. Beneath the shadow of this broad shield the segment projects laterally into another tubercle, whence issue the organs of motion, which in these Annelids are of highly curious structure. In this species they consist of a bundle of straight bristles of exquisite tenuity, and having a silky lustre; they are of unequal length, the upper ones being the longest, and the pencil diminishing pretty regularly downwards. The microscope shows them to be about one hundred in number in each bundle; and each bristle to be a delicate oar consisting of a slender stem, apparently tubular, dilated at the tip into a narrow blade terminating in a minute hook. A fibrous structure is seen to run diagonally across the blade, the edge of which is set with minute barbs pointing outwards, resembling very fine and very short bristles. The whole bundle is connected with a long slender rod of cartilage, which, doubtless acted upon by proper muscles, moves to and fro through the muscular sheath, protruding the bundle of bristles, or entirely retracting them within the body. The bristles, when protruded, slightly diverge, so that the dilated blades of these hundred oars strike with full force upon the water in rowing, if such indeed is the nature of their action, as some have supposed. In the act of withdrawal, however, into the narrow compass of the muscular sheath, the blades slide one upon another, so as to present a surface considerably diminished. But why is each bristle hooked and barbed? The obvious supposition is that these organs act like the pole of a ferry-boat in the shallows; the barbs, pointing outwards, serving to catch any roughness of the surface, and thus to push the animal by their resistance, while the terminal hook may serve to pull in an analogous manner. But then the barbed and hooked edge is the upper one of each oar, a circumstance which renders such an explanation at first sight unsatisfactory. Perhaps, however, the habit of the animal of living under stones, may in some measure account for it; the edge and point of each bristle being intended to act upon the surface of the object above it, beneath which it is forcing its way.

A secondary pencil of bristles also issues from the upper side of each of these tubercular sheaths, so as to project diagonally upwards and outwards beneath the edge of the shield-leaf. This pencil consists of between thirty and forty bristles, which are connected with a cartilaginous rod exactly similar to that of the primary bundle. These bristles run off to a very fine point, and each one is marked on both sides with serratures so delicate as to be only just discernible with a magnifying power of 300 diameters. There are thus about 270 movable bristles of exquisite structure in each segment, which multiplied by 36, the number of segments in the body, gives a total of 9720 distinct organs of motion possessed by this animal, besides the broad leaf-like shields, which perhaps may act as fins, and the short tentacular feet, one of which is placed beneath the primary bristle-sheath, on each side of the segment.

By dissection after death, I have found the proboscis in the form of a long oval sac, very muscular and firm, of a blackish hue, lying in the anterior part of the body, which, on being cut open, showed four horny teeth placed in its neck so as to meet each other in the centre. I have never seen the protrusion of this organ during life, but if it is capable of being wholly everted, as in many species, these teeth must be turned on the outside, where they would seem to be of little use. Possibly, however, the version may be only just sufficient to bring these teeth to the margin, when their action might be made very efficient by the slightest alteration in the contraction of the sac.

Some species of this genus construct for themselves enveloping tubular cases, out of mucus given off from their bodies, and fragments of shells. In these they ordinarily live concealed, but are sufficiently active when they leave their houses. Some shine with a phosphorescent radiance in the dark.

In the great class of Worms, special organs for locomotion are often altogether lacking; and, where found, are only rudimentary. Alternate swellings and contractions of certain portions of the body are a great source of progression; and these are effected by the fluids of the interior, which are driven to a given part of the cavity, and momentarily imprisoned there by the contraction of the circular tegumentary muscles before and behind it; the body swelling at that place. “The muscles of the integument are then excited to action, and the fluid is forcibly compressed forwards or backwards, according to the direction of the muscular agency. Nearly all Annelids are struck with paralysis when this fluid is made to escape from its cavity by a puncture through its external walls. The power of voluntary motion is suspended; the body of the worm becomes passive and flaccid.”

GOODNESS OF GOD.

These details convey but a feeble and imperfect notion of the numerous and elaborate contrivances which are so profusely bestowed upon these mean and grovelling worms; but they show how careful is the Creator of their well-being; how lavish of His mercies towards His meanest creatures. How unreasonable, then, is it for those to doubt His ever-watchful and unerring tenderness towards themselves, who have been made the objects of His redeeming love, and who, on the ground of redemption, “are of more value” in His sight, than all these His lower creatures put together!

FOOTNOTES:

Aplysia punctata, of which two specimens, one viewed sidewise, and the other mounting an angle of rock, showing the front of its head, are depicted in Plate XVI. Sowerby applied the name of “hybrida” to the species, and Forbes and Hanley have adopted it; but even the inflexible law of priority does not warrant the perpetuation of a name which is glaringly absurd, and expresses a manifest contradiction; for if the creature were a hybrid, it would not be a species, and not be entitled to a specific name.

Outlines of Comp. Anatomy, p. 371.

De Anim. Marin., quoted in Johnston’s Introd. to Conchology, p. 18.

This animal will be described in a subsequent chapter.

Journal of Researches, chap. i.

Johnston.

Dentalium entalis, represented at the right-hand corner of the foreground in Plate XVII.

Pileopsis Hungaricus, of which a specimen is represented in the attitude of life, adhering to the dark rock, in Plate XVII.

Litt. de la France, i. 133.