Now, if you do meet with one of these jellyfishes, be very careful not to touch it with your bare hands. And if you should happen to be bathing, and to see one floating in the water near you, just get out of its way as fast as you possibly can. For those long yellow threads which hang down from the margin of its body sting just like nettles, and the least touch from them will cause a great deal of pain. If you have a thin skin, indeed, the sting of this terrible jellyfish may make you very seriously ill, and several weeks may pass before the effects of the poison pass away. Yet the fishing-threads of this jellyfish are scarcely thicker than hairs, and the little darts which do so much mischief are so slender that you cannot see them at all without the help of a good strong microscope. Doesn’t it seem strange that such tiny weapons can be so dreadfully poisonous? PLATE XL THE SEA ACORN (2) This is a very common jellyfish indeed; yet hardly anybody ever sees it. That is because it is very small and very transparent, so that as it swims about in the water it is almost invisible. And if it is flung up on the beach it dries up in a very few minutes. But if you want to look at it, you can very easily do so. On a warm, still day, when the sea is quite smooth, just dip a small net into the water, and work it gently to and fro. Then lift it out and examine the sides carefully, and you are almost sure to see three or four little lumps of jelly, not much bigger than peas. These are sea acorns, and if you put them into a glass vessel of perfectly clean sea-water, you will very soon find that they are swimming about. For though you cannot see the animals themselves, which are quite as transparent as the water, you will notice little flashes of coloured light, sometimes blue, sometimes green, sometimes yellow, and sometimes red, which just gleam out for about half a quarter of a second, and then disappear. You might almost think that a tiny rainbow had been dissolved in the water. [Illustration: PLATE XL 1. THE STINGING JELLYFISH. 2. THE SEA ACORN.] The fact is this. Running round the oval body of the sea acorn are eight narrow bands, and on each of these are a number of very tiny scales, placed one above another, which keep on rising and falling again, like so many little trap-doors. These scales are really paddles, by means of which the animal drives itself through the water, and as they move up and down they catch the rays of light and break them up, just like that triangular piece of glass which we call a “prism.” And though you cannot see the jellyfish itself you can see these little flashes of coloured light, and so can trace the course of the little creature as it travels slowly along.