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.