2. SERPULAS IN TUBES.] On the sides of its body the serpula has tufts of little bristly hairs, just as the sabella has, which allow it to move up and down its tube. But in order to enable it to draw itself back as quickly as possible in moments of danger, it has a row of little hooked teeth on its back, by means of which it can take a firm hold of the lining of its burrow. I think you will be rather surprised when I tell you how many of these teeth there are in the row. Just fancy! Each serpula has between thirteen and fourteen thousand! If you look at the oysters in a fishmonger’s shop, you may often see the tubes of these curious worms fastened to the surface of the shells. PLATE XXXIII THE TEREBELLA (1) This is another of the worms which live in tubes. You can generally find its wonderful little dwellings by hunting in the small puddles of sea-water which are left on the sands when the tide goes out. And you can always tell them from those of the sabella and the serpula by the curious little fringe round the entrance, which is made of the tiniest grains of sand fastened together into slender threads. The tube itself is made of larger grains, and is so tough and leathery that you can give it quite a hard pull without breaking it. But as it is at least a foot long, and is nearly always carried down underneath rocks or big stones, you will not find it at all easy to dig it up. And the moment that you alarm the little animal inside it always makes its way right down to the very bottom of its tube. Sometimes a terebella will leave its tube and go for a little swim in the pool, wriggling its way through the water by first doubling its body up and then stretching it out, over and over again. But it very soon gets tired with its exertions, and sinks down to the bottom of the pool to rest. Then, after awhile, it will set busily to work, and make a new tube to live in instead of the old one. There is another kind of terebella, called the Shell-binder, which makes its tube of little bits of broken shell instead of grains of sand. You may find the ends of these tubes sticking up out of the sand about half-way between high and low-water mark. But they run down so deeply that you will have to dig very hard indeed if you want to get them out of the ground. [Illustration: PLATE XXXIII 1. THE TEREBELLA. 2. THE LUG WORM.] PLATE XXXIII THE LUG WORM (2)