Brittle starfishes are very active animals, and when they are alive their long slender rays are always wriggling and coiling and twisting about, hardly ever seeming to be still for a single moment. Indeed, one naturalist compares a brittle starfish to five very long and active centipedes stitched to a tiny pin-cushion! There are several different kinds of these very curious animals, most of which live at some little distance below low-water mark, and are hardly ever caught except by means of the dredge. But sometimes you may find one of them lying on the sand at the bottom of a pool among the rocks. [Illustration: PLATE XXXVII THE BRITTLE STARFISH.] PLATE XXXVIII THE SEA URCHIN (1 and 2) The “urchin,” as of course you know, is a common country name for the hedgehog; and the Sea Urchin is so called because it is covered all over with long spikes, just as a hedgehog is. These spines, however, are very easily broken off, and when the animal dies, and its empty shell is tossed to and fro by the waves, they are knocked off in a very short time; so that when you meet with a sea urchin’s shell lying upon the shore you nearly always find that it is covered with nothing more than hundreds of very tiny pimples. Now it is upon these little pimples that the spines grow. If you were to examine one of the spines with a magnifying-glass you would find that its base was hollow. This hollow base is just large enough to fit over one of the pimples, to which it is fastened by a strong but rather elastic muscle. So a sea urchin is able to move its spines about quite freely. Indeed, it sometimes walks with them as well as with the little sucker-feet, which it pokes out through tiny holes in the shell just as a starfish does, moving a few forward at a time, and so hitching its way along over the sand at the bottom of the sea. If you succeed in finding a live sea urchin--and you can generally do so without very much trouble, by hunting in the pools among the rocks when the tide is out--you will notice that it has a very big mouth, with five perfectly enormous teeth. They are so huge, indeed, that if you had teeth as big, in proportion to your size, they would be about as large as good big carving-knives! On some parts of the coast sea urchins are eaten as food, being scooped out of their shells with a spoon, just as we eat a boiled egg at breakfast. For this reason they are sometimes known as “sea eggs,” and those who have tried them say that they are very good indeed. You would hardly think, perhaps, that a sea urchin and a starfish could be related to one another, for they do not look in the least alike. But if you take an urchin which has lost its spines, and examine it carefully, you will see that it is really a kind of rolled-up starfish, and you will be able to count its five rays quite easily. [Illustration: PLATE XXXVIII 1. THE SEA URCHIN WITHOUT SPINES. 2. THE SEA URCHIN WITH SPINES.]