A very great deal, I assure 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 canals.
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 passes 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 passes 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 quantity 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 grass, 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 assure 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 quantity 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 glass 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 hitherto 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 classifiers to beware what they asserted in this case.
Talking of puzzling classifications, this is just the place for mentioning the batrachians, who have been made into a class by themselves, but who most distinctly belong to two classes 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 quantities 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 class 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 class to another, always improving upon her original plan without ever abandoning it.
On the shores 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 cumulate 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.