The winners in life's race

by Arabella B. Buckley

The structure of the Minnow and the living fish.

A n, nose-pit; e, eye-nerve; ea, ear-nerve; g, gills; h, heart; t, food-tube; s, stomach; k, kidney; v, vent; da, dorsal-artery. a, air-bladder; b, backbone; nv, nerve cord or spinal cord.

B n, nose; gc, gill cover; af, arm-fin; lf, leg-fins; sf, single fins; ms, mucous scales.

If you can be motionless and not frighten them you may see a good deal, for while some are dashing to and fro, others, with just a lazy wave of the tail and the tiny fins, will loiter along the sides of the stream, where you may examine their half-transparent bodies. Look first at one of the larger ones, whose parts are easily seen, and notice how every moment he gulps with his mouth, while at the same time a little scaly cover (g c, B, [Fig. 4]) on each side of the head, just behind the eye, opens and closes, showing a red streak within. This is how he breathes. He takes in water at his mouth, and instead of swallowing it passes it through some bony toothed slits (g, A [Fig. 4]) in his throat into a little chamber under that scaly cover; in that chamber, fastened to the bony slits, are a number of folds of flesh full of blood-vessels, which take up the air out of the water; and when this is done he closes the toothed slits and so forces the bad water out from under the scaly cover back into the river again. It is the little heart (h), lying just behind the gills, which pumps the blood into the channels in those red folds, and as it keeps sending more and more, that which is freshened is forced on and flows through the rest of the body. It goes on its way slowly, because a fish’s heart has only two chambers instead of four as we have, and these are both employed in pumping the blood into the gills, so that for the rest of the journey through the body it has no further help. For this reason, and also because taking up air out of the water is a slow matter, fish are cold-blooded animals, not much warmer than the water in which they are.

But while our minnow breathes he also swims. He is hardly still for a moment, even though he may give only the tiniest wave with his tail and fins, and he slips through the water with great ease, because his body is narrow and tapers more or less at both ends like a boat. At times, too, if he is frightened, he bounds with one lash of his tail right across the river; and if you look at one of the small transparent minnows you will see that he has power to do this because his real body, composed of his head and gills, heart and stomach, ends at half his length (see [Fig. 4], A), and all the rest is tail, made of backbone and strong muscles, with which he can strike firmly. This is one great secret of fish strength, that nearly one half of their body is an implement for driving them through the water and guiding them on their way. Still although the tail is his chief propeller, our minnow could not keep his balance at all if it were not for his arm and leg fins. You will notice that it is the pair of front fins (af) which move most, while the under ones (lf) are pressed together and almost still. Besides these two pairs he has three single fins (sf), one under his body, one large V-shaped one at the end of his tail, and another single one upon his back. All these different fins help to guide him on his way; but while the single ones are fish-fringes, as it were, like the fringe round the lancelet’s body, only split into several parts, the two pair under his body are real limbs, answering to the two pair of limbs we find in all backboned animals, whether they are all four fins, or all four legs, or wings and legs, or arms and legs.

These paired wings are most important to the minnow, for, if his arm-fins were cut off, his head would go down at once, or, if one of them was gone he would fall on one side, while, if he had lost his fins altogether, he would float upside down as a dead fish does, for his back is the heaviest part of his body. It is worth while to watch how cannily he uses them. If you cannot see him in the stream you can do so quite well in a little glass bowl, as I have him before me now. If he wants to go to the left he strikes to the right with his tail and moves his right arm-fin, closing down the left, or if he wants to go to the right he does just the opposite; though often it is enough to strike with his tail and single fin below, and then he uses both the front fins at once to press forward.

But how does he manage to float so quietly in the water, almost without moving his fins? If your minnow is young and transparent you will be able to answer this question by looking at his body just under his backbone, and between it and his stomach. There you will see a long, narrow, silvery tube (a, [Fig. 4]) drawn together in the middle so that the front half near his eyes looks like a large globule of quicksilver, and the hinder half like a tiny silver sausage. This silvery tube is a bladder full of gas, chiefly nitrogen, and is called the air-bladder. Its use has long been a great puzzle to naturalists, and even now there is much to be learnt about it. But one thing is certain, and that is, that fish such as sharks, rays, and soles, which have no air-bladders, are always heavier than the water, and must make a swimming effort to prevent sinking. Fish, on the contrary, which have air-bladders, can always find some one depth in the water at which they can remain without falling or rising, and we shall see later on that this has a great deal to do with the different depths at which certain fish live. Our minnow floats naturally not far from the top, and, even if he were forced to live farther down, the gas in his bladder would accommodate itself after a few hours if the change was not too great, and he would float comfortably again.

And now the question remains, What intelligence has the minnow to guide him in all these movements? If you will keep minnows and feed them yourself every day you will soon find out that they see, smell, and feel very quickly, though their hearing and taste are not so acute. They are cunning enough too, and will often steal a march upon heavier and slower fish, snatching delicate morsels from under their very noses. For our little minnow can boast of a real brain, though it is a small one in comparison with his size. All along, above his delicate backbone, the thread of nerve telegraph (nv, [Fig. 4]) runs under protecting bony arches, and sends out nerves on all sides to the body and fins; and when it reaches the head it swells out, under a bony covering, into a small brain, sending out two nerves to the ears (ea), in front of which is a second part, with two nerve-stations (e) for the eyes, and beyond this a third part, with two more for the nostrils, besides others which go to the face. Look on the top of a minnow’s head and you will see two little raised bumps (n). These are its nostrils, but remember they have nothing to do with breathing; they have not even any connection with the mouth, but are simply little covered cups, each with two openings for water to flow in and out, and they are lined with nerves, which, tickled by good or bad scents in the water, carry to the brain a warning, or a promise of good things.

Such, then, is our little minnow, and the different parts of his body are supported by a slender bony-jointed backbone, with ribs growing from it, supporting a strong mass of flesh on his sides. He is a delicate tender creature, but is protected and buoyed up by the water, out of which he never attempts to go. The thin, rounded, transparent scales which cover his body, growing out of little pockets in his skin, just like our nails on the tips of our fingers, protect this skin from the water and from rough treatment; while they themselves are kept soft by a slimy fluid which oozes out from under them, and especially through the dark line of larger scales (ms. [Fig. 4]) running along his body.