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.
In its habits the plaice is very much like the flounder, except that it does not like lying upon mud, and always chooses a spot where the bottom of the sea is sandy. And the skin of the upper side of its body, instead of growing dark brown, like the colour of mud, becomes speckled and spotted like the surface of sand. The fish is always very careful indeed to conceal itself, for even when the sea-bottom is sandy it does not lie upon the surface, but wriggles its way right down into the sand, only leaving just its eyes and a small part of its head above it.
You can always tell a plaice when you see it by the bright reddish-yellow spots upon the upper side of its body and its fins. And besides these, it always has a row of little bony knobs on the upper side of its head. You can catch it just as you can catch flounders, by paddling in the sea. But the plaice which are caught in this way are always quite small ones, for the bigger fish, which sometimes weigh as much as twelve or even fifteen pounds, live in the deeper water at some little distance from the shore.
Very often indeed, as you walk along the sea-shore, you will find a curious object which the fishermen generally call a “mermaid’s purse.” It is about three inches long and two inches wide, and is made of a black, horny substance, so tough and hard that it is very difficult indeed to tear it. And from each corner there projects a slender tube, about an inch in length. In fact it looks rather like a hand-barrow, with handles in front as well as at the back, instead of wheels.