In the first place, their colour changes. Until now, both sides of the body have been pearly or silvery white. A white fish, however, lying on yellow sand or brown mud, would be very easily seen, and some hungry creature would be sure to catch sight of it and devour it. So as soon as the little fish lies down the upper side begins to get darker, and in a very short time it is of just the same colour as the sand or mud all round it. If you look into a shallow pool in which some of these fishes are lying you will find it very difficult indeed to see them, for they look exactly like the surface on which they rest. In the second place, their way of swimming changes. When they first hatch out from the egg these little fishes swim just as other fishes do--upright, by means of their tails. For of course you know that fishes do not swim with their fins, which merely help them to keep their balance in the water. But when they lie down at the bottom of the sea they give up this way of swimming, and wriggle their way, as it were, through the water, still lying upon one side. [Illustration: PLATE IV THE FLOUNDER.] But the oddest change of all takes place in the position of the eyes. You can easily see, of course, that if a fish with its eyes in the usual place lies down on one side at the bottom of the sea, one eye is underneath its head, and is quite useless. So you might think that, except when it was swimming, it would only be able to see with one of its eyes. But a very strange thing indeed happens as soon as it lies down on the mud. The lower eye actually begins to move, and slowly travels round the head, till at last it settles down by the side of the other! That sounds impossible, doesn’t it? It is as wonderful as anything in a fairy story. Yet in every one of these so-called “flat” fishes that strange journey of the eye takes place. Next time you pass by a fishmonger’s shop just look at the soles or the flounders in his window, and you will see that in every one of these fishes the two eyes are quite close together, above the same corner of the mouth. That is because one of the eyes moved right across the head while the fish was quite small, so that it might be able to use them both as it lay at the bottom of the sea. You can sometimes catch flounders by paddling in the sea in places where the bottom is rather muddy. After a little while you are almost sure to feel one of these fishes wriggling underneath your feet, and all that you have to do is to stoop down and seize it. PLATE V THE PLAICE