HOW CRABS SEE
Perhaps, too, you would like to know something about the eyes of crabs; for these creatures see in a very odd way. On each side of the head is a kind of stalk, something like those which you may see on the heads of slugs and snails, only very much smaller. And at the tip of each stalk is a small black spot. Now if you were to put one of these little stalks under the microscope, and to look at the black spot, you would find that it was made up of hundreds and hundreds of very tiny eyes, very much like those of insects, except that instead of being six-sided they are square. So that altogether, perhaps, a crab may have three or four thousand eyes, or even more!
That sounds a very large number, doesn’t it? But then, you see, a crab cannot move its eyes up and down, and from side to side, as we can. They are fixed, and cannot be moved at all. Each eye, however, looks in rather a different direction from all the rest. Some eyes look upwards, some look downwards, some look forwards, some look backwards, and some look out on either side. So without moving its head at all the crab is able to see all round it.
Think of it in this way.
Suppose that you take a telescope and look through it. You can only see the objects at which the telescope is pointed, not the objects above it, or below it, or on each side. But if you had four thousand telescopes, fastened together in two bundles of a couple of thousand telescopes each, all pointing in different directions, and if your eyes were made in such a way that you could look through all the telescopes at once: then you would be able to see all round you, though you would only be able to look in any special direction through just one or two of the telescopes.
Now that is very much like the way in which the eyes of crabs are made. Each of these four thousand eyes is really a kind of telescope. And as they all point in different directions, the crab is able to see above it and below it and on all sides, though it only looks at any special object through one or two eyes.