But these bristles have another use besides that of a filter. Each of them is really a kind of long, slender spear with a barbed tip, which can be used as a weapon of defence. If you were to look at one of these bristly spears through a good strong microscope you would see that it was edged on both sides with sharp little hooked teeth, looking very much like those of a shark. But you need not be in the least afraid to handle a sea mouse, for although these slender spears look so formidable, they are not nearly strong enough to pierce your skin. [Illustration: PLATE XXXI 1. THE SEA MOUSE. 2. THE SABELLA.] PLATE XXXI THE SABELLA (2) A good many different kinds of worms live on the sea-shore, and one of the most curious of these is the Sabella. For it lives in long, narrow tubes made of tiny grains of sand, which it sticks together with a kind of natural glue. You may find these tubes in great numbers just about low-water mark, and hundreds and hundreds of them are often twisted up together in great masses, which are sometimes several feet in diameter. The worms can travel up and down these tubes by means of tufts of stiff little bristles on each side of their bodies; and sometimes they will leave them altogether, crawl about on the sand for a little while, and then make new ones. And if you keep them alive in a glass vessel filled with sea-water, with a little sand at the bottom, you can watch them building their wonderful tubes, carefully choosing grains of sand of just the proper size, arranging them in position just as a bricklayer lays bricks, and then sticking them firmly together. PLATE XXXII THE SERPULA (1 and 2) If you look down into the pools among the rocks when the tide is out you may often see a number of long, twisted tubes fastened to the surface of the stones at the bottom. These are the dwellings of a very curious sea-shore worm called the Serpula, and if you lift one of the stones out of the water, and look down into the tubes, you will nearly always see a bright scarlet object lying just beneath the entrance. And then you may be quite sure that the animal is alive. Now suppose that you carry the stone home with you, just as it is, and put it into a vessel of sea-water. After an hour or two you will find that the little scarlet objects have been poked out of the tubes, and that they are really tiny stoppers, just like little corks, which exactly fit the entrance when they are pulled inside. And you will also find that a plume of feathery objects, which are also bright scarlet in colour, is projecting out of the mouth of each tube. These red plumes are the gills of the worms, and they will often remain spread for hours at a time. But if you startle the animals--if your shadow falls upon them, for instance--they will draw themselves down into their tubes in about half a quarter of a second, and every tube will be corked up by its tiny stopper, just as before. [Illustration: PLATE XXXII 1. THE SERPULA.