In order to see a little more of these organs we must sacrifice a Worm ; having killed it, and divided the body in the middle, I cut off, with sharp scissors, a small transverse portion, say two or three rings, and press the fragment between plates of glass. Now, with a higher magnifying power, we discern in the midst of the translucent flesh the points in question. They are not, however, single; but each protrusile organ consists of a pair of transparent, brittle, glassy rods, shaped like an italic J] of which the recurved points are directed backwards when thrust out from the skin.

The mode in which these assist the progression of the Worm is well described by Professor Rymer Jones. " The attenuated rings in the neighbourhood of the mouth are first insinuated between the particles of the earth, which, from their conical shape, they penetrate like a sharp wedge ; in this position they are firmly retained by the numerous recurved spines appended to the different segments ; the hinder parts of the body are then drawn forward by a longitudinal contraction of the whole animal ; a movement which not only prepares the creature for advancing further into the soil, but, by swelling out the anterior segments, forcibly dilates the passage into which the head had been already thrust : the spines upon the hinder rings then take a firm hold upon the sides of the hole thus formed ; and, preventing any retrograde movement, the head is again forced through the yielding mould ; so that, by a repetition of the process, the animal is able to advance with the greatest apparent ease through substances which it would at first seem utterly impossible for so helpless a being to penetrate." *

Implements analogous to these are found in most of the animals of the class Annelida, f to which the Earth-worm belongs. But in this creature you see them in their simplest form : it is to the aquatic Worms that you must look if you wish to see the amazing diversity, complexity, and delicacy of these organs. In these there are one or two

"Gen. Outline," 202.

t From the Latin annellus, a little ring. A large class of animals known under the common name of Worms.

pairs of "feet" on each ring, consisting of wart-like prominences, which are perforate and protrusile, and through the middle of which work a number of bristles (setce), arranged in a radiating pencil, something like the hairs of a paint-brush. In this transparent and colourless little Nais from fresh water, you may see their form and arrangement; in complexity they present an advance upon the Earth-worm, for here there are some seven or eight bristles in each pencil, which radiate in the same plane, and are graduated in length ; they are very slender, bent at the tip, and as transparent as if drawn out of spun glass. It is interesting to ob- FOOT OF NAIS. serve with what lisjhtnin^-like ° rapidity they are thrust out and withdrawn in constant succession, as the body is ever lengthening and shortening.

Let us exchange this little fresh-water Worm for a marine one. Here is a Polynoe, a curious genus, very common under stones at low water on our rocky shores. It is remarkable on several accounts. All down the back we discover a set of oval or kidney-shaped plates, which are called the back shields {dorsal elytra) ; these are flat, and are planted upon the back by little foot-stalks set on near the margin of the under surface ; they are arranged in two rows, overlapping each other at the edge. These kidney-shaped shields, which can be detached with slight violence, are studded over with little transparent oval bodies, set on short foot-stalks which are perhaps delicate organs of touch. The intermediate antennae, the tentacles, and the cirri, or filaments of the feet, are similarly fringed with these little appendages, which resemble the glands of certain plants, and have a most singular appearance. If we remove the shields, we discover, on each side of the

worms. body, a row of wartlike feet, from each of which project two bundles of spines of exquisite structure. The bundles, expanding on all sides, resemble so many sheaves of wheat ; or you may more appropriately fancy you behold the armoury of some belligerent sea-fairy, with stacks of arms enough to accoutre a numerous host. But if you look closely at the weapons themselves, they rather resemble those which we are accustomed to wonder at in missionary museums,—the arms of some ingenious but barbarous people from the South Sea Islands,—than such as are used in civilised warfare. Here are long lances, made like scythe-blades, set on a staff, with a hook at the tip, as if to capture the fleeing foe and bring him within reach of the blade. Among them are others of similar shape, but with the edge cut into delicate slanting notches, which run along the sides of the blade like those on the edge of our reaping-hooks. These are chiefly the weapons of the lower bundle ; those of the upper are still more imposing. The outmost are short curved clubs, armed with a row of shark's teeth to make them more fatal ; these surround a cluster of spears, the long heads of which are furnished with a double row of the same appendages, and lengthened scimitars, the curved edges of which are cut into teeth like a saw. Though a stranger nri^ht think I had drawn copiously on my fancy for this description, I am sure, with your eye upon what is on the stage of the microscope at this moment, you will acknowledge that the resemblances are not at all forced or unnatural. To add to the effect, imagine that all these weapons are forged out of the clearest glass instead of steel ; that the larger bundles may contain about fifty, and the smaller half as many, each ; that there are four bundles on every segment, and that the body is composed of twenty-five such segments ; and you will have a tolerable idea of the garniture and armature of this little Worm, that grubs about in the mud at low-water mark.

Should it ever be your fortune to fall in with a species, of Sea-mouse (Aphrodite liystrix), which inhabits our southern coast a little way from the shore, you may be delighted and surprised with a modification of these organs, which exhibits a more than ordinarily obvious amount of creative forethought and skill. I will describe them in the words of the learned historians of these animals, MM. Audouin and Milne-Edwards :—

" The feet are divided into two very distinct branches,, the lower of which is large, conical, of a yellowish-brown hue, and much shagreened on the surface. The upper branch is much less salient than the lower. We observe at the foot of the dorsal shields two bundles of rigid bristles : the one, expanded like a fan and applied upon the shields, is fixed immediately outside the insertion of those organs; the bristles which compose it are awlshaped, without teeth, slightly curved, and directed inwards and backwards ; their colour is a clear brown, with golden reflections. The second bundle is inserted more externally, on a tuberculous foot-stalk, and points horizontally backwards and outwards. The bristles which enter into its composition are very long, very strong, and terminated by a lance-shaped point, of which the edges are garnished with teeth curved backwards towards the base. These are veritable barbed arrows, having the extremities sometimes exposed, but often concealed in a sheath which is formed of two horny pieces, capable of opening and of closing again upon them.

" The use of these two valves it is not difficult to detect. They protect the points of the arrow, and permit the Aphrodite to receive them again into its body unharmed ; whereas, without this precaution, the tissues which they traverse would be cut and mangled. But when these weapons are deeply plunged into a foreign body, as intothe soft flesh of those animals which annoy the Worm, since the sheath does not penetrate with them but foldsworms. back, it follows that their teeth are inserted without any protection, and that on account of their backward direction they can be withdrawn only with great difficulty; thus, in most cases, the dart becomes broken; but the animal is furnished with so great a number, that these losses are scarcely felt, and there remain to it amply sufficient for its defence in all contingencies."*

You will have noticed that the learned French zoologists seriously countenance the notion that these exquisitely elaborate organs are weapons of offence. But in this I think they are in error, misled by the resemblance, already alluded to, which the parts bear to weapons of human construction. The manner in which they act as implements of locomotion has been beautifully demonstrated by Dr. Williams in the ISTereidous Worms, of which he observes that in nearly all species the feet are constructed with express reference to progression on solid surfaces. In many instances, the bristle is compound, consisting of a staff with a variously armed point or blade jointed to its extremity. "Viewed by the light of mechanical principles, nothing cau be so obvious as the reason why the setce in these, as in nearly all other Annelida, are jointed. If they consisted of rigid, unbending levers, it is manifest that they would prove most awkward additions to the sides of the animals; if fixed too deeply in the surrounding soil, they would not act at all as levers ; if too superficially,, the Worm would be compressed in its tube at the moment when the setce of the opposite feet would meet in a straight line. These difficulties are effectually and skilfully obviated by the introduction of a joint or a point of motion on each seta. This is one instance among many which the eye of the mechanician would detect in the organisation of the Annelida, in which Nature takes adroit advantage of mechanical principles in the attainment of her ends." t * " Litt. de la France," ii. 71. + " Kep. on Brit. Annelida," 211..

Look now, in illustration of these principles, at the bristle-feet of this beautiful green Phyllodoce. No doubt you have often seen it in the little hollows of our rockyledges, and especially on beds of young mussels; and probably you have admired the elegant ease with which its lithe and tortuous body writhes and winds, like a bit of green silken cord, in and out among the compactly crowded shells. You have wondered, too, at the difficulty which attends the attempt to take it up, not on account of the rapidity of its motions, but because of the extreme slenderness and slipperiness of the subject, and of the power which it possesses of insinuating itself into the smallest crevice.

The foot in this genus has but a single branch, and a single pencil of bristles, which is placed between the flat swimming leaf that ornaments each segment and the lower cirrus. The bristles are of the compound jointed form, but the joint is fixed in a peculiar manner. The basal portion is drawn out into a very slender, long, straight shaft, terminating in a knob somewhat resembling the end of a limb-bone. This is slit in one direction to receive the terminal piece, which is shaped somewhat like a lancehead, and is inserted into the slit exactly as a knife-blade is fixed into the haft. The head is in fact a knife-blade, with a thickened back and a very thin edge, which is notched into teeth of the most exquisite delicacy. The blade is slightly curved, and drawn out into a long acute point; and the w^hole bristle is formed out of an elastic horny substance (probably chitine), that rivals in transparency and brilliancy the purest flint-glass.

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