Well, one day I went into a house where there was a great glass tank. It was like a glass room full of water. As I stood looking in through the glass side of this tank, I saw, gliding through the water, a large, lovely creature, in silver and rose color. It had big black eyes, set in a golden ring. This creature seemed to move without the least effort. It made no sound. It slid by me like the figures in a beautiful dream. It rose. It turned. It sank,--and all without seeming to exert itself in the least. I saw no effort at motion; but now and then a tremor of a forked tail, and now and then the gentle wave to and fro of a pair of gleaming, thin, silken things, like fans, half unfurled. This, then, was a fish at home! Here was a fish living in the water, and doing as it pleased. The clear water added to its beauty. It shone like a gem. Other creatures, as beautiful, but different, lived with it. They crossed and recrossed each other’s course. They left no track. They had no path. They moved on, far more easily than a bird moves in the air, or a man on the land. From that day I loved the fin family. Let us look at this family. I told you that a bird is built on the plan of a boat to sail in the air. The fish, also, is built on the boat plan. He is a boat to move not _on_, but _in_, the water. Perhaps this is not a fair way of speaking; for birds and fish were made long before men. And no doubt men built their boats on the plan of the water-birds and the fish. The fishes vary much in shape and size. We will speak now of the ordinary fish type. We will take a model or pattern fish, such as the perch or the mackerel. The head is sharp and wedge-shaped. It serves as the prow of the boat, to divide the water. The body is long, narrow, smooth. It has scales lapping over each other, and are all turned from the head toward the tail. The tail is like the rudder of a boat, to direct its course. It serves also as the paddle, or propeller, to drive it through the water. There is a long slim fin on each side, just below and behind the head. These serve as balances, and, also, sometimes as oars. There is a long fin, sometimes there are two fins, on the ridge of the back. There are also two fins on the lower line of the body. These steady the fish in the water. They help to keep it, as you would say, “right side up.” There are two more fins under the body, near the tail. What you call the tail is really a fin. The true tail is the narrow end of the body, just above this final fin. Let us look at the head. Just on the front is the mouth. It has plenty of teeth. I shall tell you of these teeth in another lesson, when I tell you about the scales. Just above and behind the mouth are the eyes, one on each side of the head. They never have any eyelids.