LETTER x.x.xVI.

PISCES. (_Fishes._)

We are becoming terribly learned, my poor child, and I am half afraid you will be getting tired of me. When I was little myself, I had rather a fancy for breaking open those barking pasteboard dogs you know so well; to see what was inside them. Why should you not, then, feel a certain amount of interest in looking with me into the insides of real animals? Still I cannot conceal from myself that the subject grows very serious at last, and that while I am busied in struggling to make myself intelligible through the endless crowd of facts which surround me, I am apt to neglect chatting with you as we go along. Happily, however, here is an opportunity for so doing.

Up to the present time we have lived, as it were, upon the explanations I gave you whilst studying the action of life in yourself, and all the organs we have met with since, have been only, properly speaking, reproductions, more or less exact, of those which you yourself possess.

But, in pa.s.sing over into the kingdom of fishes, we find ourselves in the presence of something altogether new, and I must go back to our old familiar style of talking to open the subject.

Take a water-bottle half-filled with water, and shake it well, and you will see a quant.i.ty of white froth come to the surface of the liquid.

This is the air which having been drawn in by the water, as it went up and down in the bottle, is now struggling to fly off again in bubbles as fast as it can. But the whole of it does not get away; a small portion remains behind, and melts, as it were, into the water, as a morsel of sugar would do, taking up its abode therein. This seems odd to you, but I will tell you how you may convince yourself of the fact.

Get a small white gla.s.s bottle, slightly rounded, and thin at the bottom, if possible; fill it with water, and hold it for a short time over a lighted taper. If you do this carefully there is no danger. You will soon see tiny little b.a.l.l.s, looking like drops of silver, rise from the bottom of the bottle, come up to the surface, and burst. This is the air which was installed in the water, as I described above, and which is now running away from the heat of the candle, as the inhabitants run away from a house on fire. After a time the whole will have pa.s.sed off, and the little b.a.l.l.s will cease to rise.

But what has all this to do with fishes? you ask.

A very great deal, I a.s.sure you, dear child. If there had been a little fish in your bottle, before it was exposed to the flame, it would have found means to make use of that air, whose original presence in the water you cannot refuse to believe after having seen it come out. It is with this air that fishes breathe in the water. They do so rather feebly, I admit; but, as if to make up to them for the small amount of the air placed at their disposal, it contains more oxygen than that we breathe ourselves, because oxygen, dissolving more readily in water than nitrogen, is there in greater proportion. Of course, you do not suppose that fishes have lungs like ours? I dare say you know the two large openings on each side of their head, called _gills_, by which the fishermen string them together to carry them away more easily? It is there you will find their lungs, to which the name of _branchiae_, or gills, has been given, because they are so different from other organs of respiration that it was impossible to use one word for the two. The arrangement of the gills varies considerably in the different species, but their general form is the same everywhere. They are composed of a number of plates, consisting of an infinitude of leaflets, arranged like a fringe, and suspended by bony arches, into which plates and leaflets the blood pours from a thousand invisible ca.n.a.ls.

First of all, then, we must see how blood circulates in fishes.

Like reptiles, their heart has only one ventricle, and yet the arterial and venous blood go each its separate way without the slightest risk of being mixed; but this is because fishes have not that double system of veins and arteries which hitherto we have always met with. The venous blood goes to the heart, which drives it into the gills, from whence it pa.s.ses forward of its own accord, as arterial blood into the organs, under the remote influence of the original impetus from the heart, the newly-arrived blood incessantly driving the other before it into the vessels of circulation. It does not flow very quickly, as you may suppose; and as the heart is close to the head, its action is but very feebly felt at the extremity of the body, when this happens to be very long. Nature has, in consequence, taken pity on the eel, whose tail is so far from its heart, and provided accordingly. Dr.

Marshall Hall has discovered near the tip a second, reinforcing heart, so to speak, which has its own pulsations, independent of the pulsations of the one above, and gives a fresh impetus to the sluggish blood, [Footnote: Many observers refer this to the lymphatic system.--TR.]

which otherwise, as it would seem, would scarcely be able to accomplish the long return journey. Finally, even with an additional heart in thetail, the circulation among fishes is quite on a par with their respiration. They have a melancholy steward, whose legs are very heavy, and his pockets very light, and their life comes down a peg lower in consequence. It is always the same life nevertheless--you must never lose sight of that fact: it gets low in consequence of the imperfection of the machine, but without changing its nature, any more than the light in our different sorts of lighting apparatus. You remember that comparison of the lamp with which I began my story, and which you could not at the time see the full value of? From a dungeon lamp up to a candle, you have always grease burning in the air at the end of the threads of a wick. It does not burn equally well everywhere, and does not always give the same amount of light; but that is all the difference. From the mammal to the fish, it is always hydrogen and carbon (as we have said of the grease) which oxygen sets on fire in the human body at the fine-drawn extremities of the blood-vessels; only the fire is lower in some than others, and the life with it. Let us now look at the circulation of water in the fish's body.

The gills communicate with the mouth by a sort of grating, formed by the bony arches to which the gill-plates are suspended. The fish begins by swallowing water, which then pa.s.ses through the grating and circulates round the innumerable leaflets of which each plate is composed, and among which creep the blood-vessels. It is through the thin coats of these leaflets that the mysterious exchange is made of the unemployed oxygen in the water and the carbonic acid in the blood.

When this is over, the cover which closes the gills opens to let out the water, and a fresh gulp takes its place; and so on continually.

When the fish is out of the water its gills fall together and dry up; the course of the blood, already so weak, is interrupted by the breaking down and shrinking of the vessels, and the animal can no longer breathe; so that we have here the curious instance of a creature breathing oxygen like ourselves, who is drowned, if we may use the expression, in the air in which we find life, and lives in the water in which we are drowned. While he is in the water matters take another course, and his gills, moistened and supported, accommodate themselves perfectly to the contact of the air, which desires nothing better than to give up its oxygen to the blood, through the coats of the capillaries.

Accordingly you will often see fishes--carps, for example--come to the surface of the water to inhale the air like a mammal or a reptile.

This is a valuable resource, which supplements the parsimonious allowance of air given out to them by the water. There are even certain fishes whose gills, more firmly closed than those of others, have, in addition, a number of cells, which retain for a considerable time a sufficient quant.i.ty of water to preserve the gills in their natural state. These fishes can easily take an airing on land, where they breathe the air as you or I do, and are downright amphibians.

The most celebrated of these is the _Anabas_, or "climbing-fish."

an Indian fish, which not only can remain many days out of the water, but also amuses itself by climbing up the palm trees--it is hard to say how--and establishing itself in the little pools of water left by the rain at the roots of the leaves. But we need not go to India to find those wandering fishes. There is one of them living among ourselves who can walk about in the gra.s.s, and I was talking to you about him only just now--that is the eel. If you ever put eels in a fish-pond you must, I a.s.sure you, try to make it agreeable to them, otherwise they will have no scruple in setting politeness at defiance and moving off to seek their fortune elsewhere. In a country walk, when the dew is on the ground, you yourself may chance to come across one or two of these gentlemen, who have had their reasons for changing their residence, and whom you will see gliding so briskly along that they will deceive you into taking them for snakes if you have not a very experienced eye; so much so, that in certain parts of France where the peasants ate snakes formerly, they reconciled themselves to the sickly idea by christening them _hedgerow-eels_.

On the other hand, fishes may be drowned in water just as easily as ourselves if it does not contain air. The little fish who could have lived very well in the bottle we were just now talking about before you exposed it to the flame of the taper, would have died in it after all the air-bubbles had gone off; and I hope I need not tell you why.

In the same way, if you leave fishes too long in a small quant.i.ty of water without renewing it, they suffer exactly as we do if the air which we breathe is not changed often enough. As soon as they have consumed what oxygen is in the water, it can no longer keep them alive.

It is then, especially, you will see them come gasping to the surface to call upon the air for help. Those who keep gold fish in a gla.s.s bowl ought to know this, and to change their water oftener than is generally done. When we take poor little creatures from their natural way of life, and set a human providence over them in the place of the Divine one which has. .h.i.therto been their safeguard, the least we can do is to acquaint ourselves with the laws of their existence, so that we may not expose them to the risk of suffering by our ignorance.

Finally, there are fishes whose gills, still more greedy of oxygen, will not act well except in thoroughly aerated water, and who would soon die in our tanks. This is the case with the trout, who is only happy in the waters of hilly countries; rich with all the air they have carried along with them as they fell from rock to rock. Now that people are beginning to do with fishes what has long since been done with sheep and oxen--keep them in flocks to have them always ready for use--you may perhaps hear a good deal said about vessels made expressly for the carriage of trout, with a thousand inventions besides for sending air into the water, and you will not have to ask the meaning of this now.

I promised last time that I would revert in the chapter of fishes to that marvellous transformation of the crocodile which has been explained by the torrent of water he draws into his stomach. You could understand nothing about it the other day; but after what we have just seen the explanation suggests itself. Just as the extraordinary activity of life in birds is explained by that double oxygenization of blood, of which part takes place in the lungs and part in the reservoirs of air placed everywhere in the way of the capillaries, so this sudden increase of energy in the crocodile the moment it plunges into water may be explained by a second respiration suddenly established in the vast cavity of the abdomen, by the contact of the capillaries with the water which penetrates there. Hence the crocodile would then have, like the bird, a double respiration: only with him the one would be permanent and from the lungs, the other temporary and from the stomach. By this, on the one hand, he would rise up to the birds, since the blood encounters air twice over in its course, while, at the same time, he would plunge into the world of fishes, since the blood has to seek air in the water. The above, be it remembered, is only a supposition, and I ought to add that in this case there would be a good deal of danger in observing nature at work, for in front of the laboratory, where she is toiling in secret, stands on guard a row of teeth, by no means encouraging to indiscreet intruders. At the same time, if there ever were a legitimate conjecture, this is it. Everything seems to confirm it; and if it be true, we should have in the crocodile a specimen of each of the four systems adopted by nature for the mammal, the bird, the reptile and the fish. At first I spoke of two, then of three; so that even in my addition I was modestly below the mark, and had really some grounds for recommending our friends the cla.s.sifiers to beware what they a.s.serted in this case.

Talking of puzzling cla.s.sifications, this is just the place for mentioning the _batrachians_, who have been made into a cla.s.s by themselves, but who most distinctly belong to two cla.s.ses at the same time; not like the crocodile by details borrowed from each, but by a fundamental change which takes place at a certain period in their organization. The batrachians are in reality reptiles, but they are reptiles which begin by being fishes, and real fishes too.

If you have ever strolled about in the country, you must have often come across those great pools of water which collect at rainy seasons in the ruts of deep lanes. Amuse yourself by looking into them in early summer, and unless the land is too parched and dry, the chances are that you will see quant.i.ties of little black fishes, almost entirely composed of a long tail joined to a large head, playing jovially in the muddy waters, and looking as if they had dropped there from the skies. These are young frogs--_tadpoles_, as we call them--and they are beginning their apprenticeship of life. Enclosed in each side of those great heads, they have gills, and they breathe in the same manner as fishes. Presently the two hind feet begin to bud out and grow, little by little; then the fore feet; finally, the tail wastes away till it disappears; and thus insensibly the tadpole is transformed into a frog. Observe here that the tadpole's gills share the same fate as his fish-tail; they wither and disappear by slow degrees, and gradually as they do so, his lungs are developed. The animal changes his cla.s.s very quietly, and without ceasing to be genuinely the same, although it would be impossible at last to recognize the old individual in the new if you had not heard its history beforehand. This is one of the most striking exemplifications I know of the mysterious process by which nature has insensibly raised animals from one cla.s.s to another, always improving upon her original plan without ever abandoning it.

On the sh.o.r.es of certain subterranean lakes which exist in Carniola, a country subject at this time to Austria, there are to be found batrachians far more ambitious than our frog--namely, the _proteans_.

These c.u.mulate rather than change: they become reptiles without ceasing to be fishes, if I may so express it; they develop lungs as they grow up, and yet keep their gills. I could tell you a thousand other particulars about these batrachians if I were to examine them all in succession; for it is a very motley family, in the bosom of which the transition from reptiles to fishes is in some imperceptible manner accomplished; from the frog, which the unanimous consent of mankind has always ranked among reptiles, to the axolotl or siren, who lives in Mexican lakes; and who, feature for feature, is exactly like a carp, with four little feet fastened under him. To be quite in order, the batrachians ought to have followed the reptiles, for their interior organization is the same; but how could I tell you about their gills without explaining that there was air in the water? and I did not want, for the sake of these intruders, whose babyhood-gills only just appear and disappear, to rob the history of the fishes of its most interesting points.

Let us be satisfied, then, with this pa.s.sing glance at a dubious cla.s.s, whose history is only a repet.i.tion of two others, and let us return to our friends the fishes. We have seen how they breathe, now let us look how they eat.

The modifications of the digestive apparatus are endless among fishes.

The lampreys, who are placed in the lower ranks of the cla.s.s, carry out to its fullest extent the type which we have already seen indicated in the serpent. The digestive tube is quite straight, without any perceptible swelling, and does not even go the whole length of the body. It comes to an end at some distance from the tail. Among some fishes an odd tendency begins to display itself, which we shall meet with again farther on. The digestive tube, after going downwards towards the bottom of the body, as we have seen it do so constantly hitherto, doubles back, and comes up again to the throat, under which it empties itself. In most cases the stomach is distinct; but it a.s.sumes a thousand different forms; as if nature had wished to try her hand in all sorts of ways in the construction of these imperfect vertebrates, before adopting the definite model which was to serve for the others.

The liver is enormous, and generally contains a great quant.i.ty of oil, the taste of which you will know if you have ever swallowed a spoonful of cod-liver oil; but in most fishes its old companion, the _pancreas_, has disappeared. In its stead you will find, close by the outlet of the pylorus, the open ends of certain small tubes, which are shut in at their upper extremity like a "blind alley," and through which descends into the interstices a thick glairy fluid, given out from their sides or walls. The result is the same, you see, although the organ is different; and, remarkably enough, these little tubes are wanting among fishes, which, like carp, have a species of salivary glands in their mouths, of which the others show no trace; from which one may fairly conclude that these glands and tubes mutually supply each other's places. Here, then, you see an instance of the light which different animal organisations throw upon each other when they are compared together. In fact, this one establishes pretty clearly the real office of the pancreas in the higher races, exhibiting it to us as an internal salivary gland, intended to complete the work only begun by those in the mouth, in the case of lazy people who swallow their food too quickly.

There is the same diversity in the mouth as in the intestine. Some fishes, like the skate, have no tongue at all. Others, instead of a tongue, have a hard dry filament, very nearly immovable, and which one would think was put there like a stake, to show the place where the tongue is to be found in the more perfect organisations. There are even fishes, like the perch and the pike, whose tongue is furnished with teeth, or rather fangs; an evident sign that it has forfeited the confidential position occupied by your own good little porter. You must know also that the perch and the pike, like many other of their fellows, have teeth all over the mouth. This invasion of the palate by teeth, which began in the lizard and the serpent, a.s.sumes alarming proportions here. It is not merely the roof of the palate which is spiked with teeth: above, below, at the sides, everywhere to the very limits of the oesophagus, the little fangs triumphantly stick out their slender points. It is impossible, therefore, to state their number.

Nature has scattered them broadcast without counting, just as she has done with the hairs of the beard round the human mouth; and the comparison is not so impertinent as you may think. They sometimes form an actual internal beard, even thicker than our outer one, and which sprouts from the skin into the bargain. There is one fish whose teeth are so delicate and so close together that, in pa.s.sing your finger over them, you would think you were touching velvet. This does not refer to the shark, mind. His teeth are sharp-cutting notched blades, hard as steel, arranged in threatening rows round the entrance of his mouth, and cut a man in two as easily as your incisors do a piece of apple. Others, such as the skate, have their mouths paved--that is the proper term--with perfectly flat teeth. The first time your mamma is sending to buy fish beg her to let you have a skate's head to look at.

You will be interested to see the small square ivory plates laid close adjoining each other, like the tiles of a church floor. It is in fact a regular hall-pavement, over which the visitors glide untouched, and are then swallowed down in the lump; thus entering straight into the house without having been stopped by the inscription nature has placed over your door and mine--"Speak to the Porter."

But all this is nothing compared to the lamprey's entrance-hall, which differs from ours in quite another way. The lamprey, as I have already told you, ranks almost lowest among fishes, and consequently among vertebrate animals, of which fishes form the rear-guard. Indeed, it is almost stretching a point to consider her worthy to bear the proud t.i.tle of a vertebrate at all; for the vertebral column, so clearly marked in other fishes, where it forms the large central bone, is only faintly indicated in certain species of lampreys, by a soft thread (or filament), which is rather a membrane than a bony chaplet, and at the top of this mockery of a vertebral column is the creature's mouth. If you ever had leeches on, you will remember the sharp sting you felt when the little beasts bit you. Well, the lamprey feeds herself just in the same way as the leech does. Her mouth forms a completely circular ring, which sticks to the prey, and through which runs backward and forward a small tongue armed with lancets. This darts out to pierce the skin, and draws in the blood as it retreats. Round your lips well; dip them so into a gla.s.s of water, and draw back your tongue, and you will at once feel the water rise into your mouth. It is by a similar sort of proceeding that leeches relieve people of the blood they want to get rid of; and in the same way the lamprey draws out the blood of the animals upon which she fastens.

What a long way we have come already! How very far we find ourselves here from the little mouths we first talked about as chewing their eatables so prettily! With the lamprey we bid adieu to the cla.s.s Vertebrata--the n.o.bility of the animal kingdom--among whom nevertheless we must distinguish between the peer, who approaches nearest the person of his sovereign, and the inferior provincial lords who live at a hundred miles' distance. There is only one step from the lamprey to the _mollusks_ or soft-bodied animals, and this is the course which animal organisation seems really to have taken in its progress.

But nature never moves forward in a single straight line. In pa.s.sing from the mollusk to the fish to get thence to the higher vertebrates, she turned aside in another direction toward a cla.s.s of animals which rises far above mollusks, but which leads to nothing beyond.

One would think there had been a check here, as if the creative power, having discovered that it was going in a wrong direction, had retraced its steps; if it be allowable to apply common ideas and expressions to our conceptions of that Great Intelligence which has arranged the plan of the mysterious ladder of animal life.

The animals we must examine next, on account of their superiority to the rest, are insects. Small as the ant is, it would not be right to let her be preceded by the oyster.

LETTER x.x.xVII.

INSECTA. (_Insects._)

Before speaking of insects, my dear child, it will be necessary, in the first place, to tell you to what primary division they belong and on what characters this division has been established. And here I find myself in a difficulty. We have been but too learned already, and now we run the risk of becoming still more so, if we commence an attack on the three primary divisions which follow the vertebrates. We shall have to encounter terrible names and tedious details, besides having to take into account a thousand things of which we have not yet spoken.

We are going on quietly with the history of the feeding machine which occupies the middle of the body, and learned men never looked in that direction for the establishment of their divisions; between ourselves, it was not accommodating enough. They have fallen back upon the locomotive apparatus (_movement machine_) which affects the body all over, and which they have proclaimed to be the leading feature of the animal organization, without noticing however that it is, after all, but the servant of the other. It is true that the great divisions are more easily established upon this point than the other, because the differences are more decided. It separates what the other unites, and thus it is that nature carries on that beautiful combination which the Germans have so accurately named "_Unity in Variety_" that is to say, she is always at work, as I have already told you, on the same canvas, but always embroidering it with a different pattern.

Wait! I have something to promise, if you are very good, and if this history (that of the feeding machine) should have given you a taste for inquiry. I will tell you another time the history of the movement machine, and there the cla.s.sification of our learned men will come in naturally very well. In the meantime we will do as they do, and just shut our eyes to their divisions, in which the feeding machine can have no interest, because they were established without reference to it. We will content ourselves, then, without further pretension to science, with modestly examining the last transformations of our pet machine in the princ.i.p.al groups of the inferior animals; of which groups I will now tell you the names in their proper order. They are as follows: Insects, Crustaceans, Mollusks, Worms, and Zoophytes. You must take these names on trust; those which you do not understand will be explained in their places.

1. _Insects._--I know not where it was I once read that there are said to be something like a hundred thousand different species of insects; and I verily believe this is not all. Of course we shall not attempt to review the whole of this formidable battalion. Let us take one of those you are most familiar with--the c.o.c.kchafer, for instance--and examine what goes on in his inside. The history is nearly that of all the others.

"Fly away, c.o.c.kchafer, fly!" says the song; and surely it is a bird that we have here, and a bird which will appear to you even more wonderful than those of which I have already spoken, when you have considered the simplicity, and at the same time the strength, of his organization. His mode of flight is rather lumbering, it is true; he is, in comparison with the large flies, what the ox is to the deer; but when you contrast the weight of his thick body with the delicacy and narrow dimensions of the two membranes which sustain him in the air, you may well ask yourself how those little morsels of wings, thin as gold-beater's skin, can carry such a ma.s.s along. In fact, they only accomplish this feat of strength by dint of an excess of activity almost startling to think of. When you run as fast as you can, how many times, think you, do you move your legs in one second? You would be somewhat puzzled to say; and so should I: but I defy you to count ten. Now the bird makes his wing move much oftener when he beats the air with rapid blows as he flies; but even he does not strike a hundred strokes in a second: and what is this to the feats of the c.o.c.kchafer's wing? It is not hundreds but thousands of times that he flaps his wings in a second; and here let me hint, by-the-by, that when people seriously wish to find out a method of travelling in the air, they will lay aside balloons, of which they can make nothing in their present condition, and will set to work to fabricate machines with wings which shall beat the air as fast as those of the c.o.c.kchafer. This sounds extravagant, but I have seen an electric pile fixed in a stand with gla.s.s feet, which caused a little hammer to beat thousands of times in a second: and surely the hammer could have been made to communicate its movement to a small wing! Forgive me this little castle in the air! The idea came into my head a long while ago, and the c.o.c.kchafer has just reminded me of it. I will not, however, pursue the subject, neither will I offer to explain the method used for counting the beats of an insect's wing.

That would carry us farther than would be desirable.

To return to our little animal. I leave you to imagine the enormous amount of strength required for such precipitate motion. We have spoken of the rapid course of the blood in birds during flight: who shall calculate its comparative rate in this fabulously wonderful locomotive, the c.o.c.kchafer? And if we lift up the cuira.s.s which encases it, what do we behold? Not a single trace of all the complicated circulation-apparatus you have learnt to know so well; neither heart nor veins nor arteries; only a quant.i.ty of whitish liquid, equally distributed throughout the whole internal cavity. Not a trace of lungs, nor any apparent means of renovation for this seemingly motionless blood; for blood it is, in spite of its color, or, at any rate, blood in its first stage of formation. It also has its globules--ill-formed, it is true, and altogether in b.a.l.l.s--like those found in the chyle with us; which chyle, be it observed, is the same color as the blood of insects, and may also be considered blood in its apprenticeship.

By what magic, then, is this raw, imperfectly-formed steward, who seems altogether stationary, enabled to accomplish exploits which would stagger his higher-bred compeers, agile and perfected as they are?

Where does he pick up the oxygen necessary for such repeated movements, it being an established fact that no animal can move at all without consuming oxygen, and that the quant.i.ty consumed is in proportion to the rate of motion? Look under his wings for an answer. There, all along his body, you will observe a number of small holes, pierced in a line, at regular distances, and furnished with shutters of two kinds.

They are the mouths of what are called _tracheae_, or breathing tubes: and from them branch out a mult.i.tude of little ca.n.a.ls, which, spreading in endless ramifications through every part of the body, convey to the whole ma.s.s of the blood, from all directions, the air which makes its way into them through the tracheal holes. In this case, you see, it is not the blood which seeks the air, but the air which seeks the blood; whence arises a new system of circulation, whose action is all the more energetic because it is unintermitting, and makes itself felt everywhere at the same time. A little while ago we were wondering at the twofold respiration of birds; yet this is far less surprising than the universally-diffused respiration of insects, who may well be able to do without lungs, seeing that their whole body is one vast lung in itself.

For the rest, do not trust to appearances, nor imagine that the blood of our friend the c.o.c.kchafer in reality remains motionless around the air-tubes, idly drinking in the oxygen which is brought to it. Though not flowing in enclosed ca.n.a.ls, it is not the less continually displaced by regular currents, which sweep through and renew this apparently stagnant pool. Nor is this the only instance of such a current presented to us by nature.

Guess, however, if you can, where you will have to look for the counterpart to the circulation of the c.o.c.kchafer. In ocean itself!

But, remember, nothing is absolutely little or great in nature, who applies her laws indifferently to a world as to an atom. The blood of our world is water, which contains in itself all the germs of fertility, and without which, as I have already told you, life is impossible either in the animal or vegetable kingdom. The water of brooks, streams, and rivers, flows along in channels, which, when figured in a map, present to the eye of the beholder an exact picture of the system of circulation found in the vertebrated animals. But the waters of the sea are borne along, like the blood of insects, by a secret circulation, which cannot be represented on the map; _i.e._ by immense currents everlastingly in action, some on the surface, some in the mid-heart of the ocean, which drive it in ceaseless course from the equator to the poles, from the poles to the equator; so that the Supreme Intelligence, in His overruling providence, has ordained the same law to set in movement the immensity of ocean, and to effect circulation in the c.o.c.kchafer's few drops of blood. In the latter we find the moving agent to be a long tube, which runs the whole length of the back, and is called the dorsal vessel (from the Latin _dorsum_, back). I told you that the c.o.c.kchafer had no heart under his cuira.s.s, but I spoke too hastily. The dorsal vessel is a _true heart_, but a heart devoid of veins or arteries, and thrown into the midst of the blood. It dilates and contracts like ours, sucks in the blood by means of side-valves, which act as our own do, and drives it back again into the ma.s.s by that valve at its extremities, which opens near the head.

From thence arises a continued to-and-fro movement, which sends the blood from the head to the tail, and brings it back again from the tail to the head. But who would recognise, in this simple primitive organisation, where all seems to go on of its own accord, as it were, the same machine, with all its complicated movements, that we have been so long considering?

Well, in this apparently universal shipwreck of all the organs we know so well, there is yet one which survives, and remains the same as ever, namely, the digestive tube. I began by saying the insect is a bird.

His digestive tube is formed upon the same pattern as that of birds, so that naturalists have bestowed the same names on the various parts in each of them. After the oesophagus comes a crop (_jabot_), very distinctly indicated; then a gizzard with thick coats, in which the food is ground down. The hen, if you remember, swallows small pebbles, which perform in her gizzard the office of the teeth in our mouths.

The c.o.c.kchafer has no need to swallow anything. His gizzard is furnished with little pieces of horn; real teeth, fixed in their places, which have a great advantage over the chance teeth picked up at random by the hen. I pointed out to you in birds, between the crop and the gizzard, a swelling or enlargement of the digestive tube, pitted with small holes, where the food is moistened by juices. The same enlargement is found here, covered all over with a mult.i.tude of small tubes, which might easily be mistaken for hairs, from which also falls a perfect shower of juices. The only difference is, that it comes after the gizzard, instead of before it, as in birds. Some naturalists, considering that the manufacture of chyle takes place here, have called it the _chylific ventricle;_ [Footnote: The corresponding protuberance of the birds bears a name, somewhat similar, but stillmore barbarous. I had pa.s.sed it over in silence, because, I make the confession in all humility, I do not understand it; but a remorse now seizes me: it is called the _Ventricule succenturie._] a somewhat barbarous name, but one which explains itself, and might with truth be applied to the _duodenum_ of the higher animals. Bile is poured in close to the hinder end of it, but you must not look for the liver; it has disappeared, or rather its form is entirely changed. You remember what the _pancreas_ had become in fishes; _i.e._ a row of tubes giving out a _salivary fluid._ Such is exactly the appearance of the liver in the c.o.c.kchafer.

Instead of that fleshy substance on which hitherto the office of preparing the bile had devolved, you see nothing but a floating bundle of long loose tubes, which, opening into the intestines, pour in their bile. The organ is transformed, but we recognise it again by the office it performs, which continues the same. As to the _pancreas_ it is wanting here, as in the fish with salivary glands; but in its place in many insects other tubes, acting also as glands, pour saliva into the _pharynx; i. e._, the cavity at the back of the throat.

As you see, therefore, everything is found complete in this tube of a few inches long; and you can also distinguish there a small and a large intestine. We are speaking of the c.o.c.kchafer, which feeds on the leaves of trees; and it is for this reason I name some inches as the length of the digestive tube. This would not be longer than the body itself, had it been destined, as in the case of many other insects, to receive animal food. In fact, the law which we have shown to exist with regard to the ox and the lion, rules also over the insect-world; and whilst a radical change seems to have been made in the rest of the organisation, here everything is in its place, and we find ourselves in the same system.