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.