But there are round-mouthed fishes even more greedy than these. It is not only among the lower forms of life that some creatures, such as worms, which are driven from the outer world, find a refuge inside other animals. But here again we meet with the same thing, for those relations of the lampreys, the hags or borers, which we mentioned above, use their sharp teeth to bore their way into other fish so as to feed upon them. These greedy little creatures actually drill holes in the flesh of the cod or haddock and other fish, and eat out the inside of their bodies, so that a haddock has been found with nothing but the skin and skeleton remaining while six fat hags lay comfortably inside. So the round-mouthed fishes, feeble though they are, hold their own in the world. How long ago it is since they first began the battle of life we shall probably never know for certain; but if some little horny teeth[9] found in very ancient rocks belong to their ancestors, they were most likely among the first backboned animals on our globe. At any rate they are very interesting to us now, for they have wandered far away from the true fishes, and give us a glimpse of some of the strange by-paths which the backboned animals have followed in order to win for themselves a place in the race for life. [Illustration: THE ANCIENT FISH & THEIR HUGE RIVAL] NOTE.--For description of the Picture-Headings see the Table of Contents. CHAPTER II. HOW THE QUAINT OLD FISHES OF ANCIENT TIMES HAVE LIVED ON INTO OUR DAY.