Now, at first sight we might imagine that these huge monsters, the terrible tyrants of the sea, must be the last and most finished production of fish-life; but if we look a little closer we shall be undeceived. Examine a shark in any good museum, and you cannot fail to be struck with his strange form. Look first at his tub-like body, so different from the narrow wedge-shape of the minnow, the herring, or the salmon. Then observe his skin, which is either tough, more like that of other animals, or thickly covered with short blunt teeth, which sometimes, especially in front of the fins, become long pointed spines. There is no trace of fish-scales here. Look at his mouth opening under his pointed snout, and you will see that as the skin turns over the lips these blunt teeth line his mouth, so that he has several rows fit for biting, and they are sometimes so formidable that they can cut a man in two at one snap. Then look more especially at the sides of his throat, and there you will see on each side from five to seven slits, reminding you at once of the slits of the lamprey, though they are long instead of round. For the shark has never arrived at having true gills under a horny cover like the minnow, but still breathes by pouches and slits somewhat after the way of the lowly round-mouthed fishes. Lastly, observe his curious tail. In nearly all living fish the tail is even[11] or V-shaped, but in the sharks the top point is usually longer than the lower one,[12] and in some, such as the Thresher, it is very remarkably so. This uneven tail is the badge of a very ancient race; out of the shark family we scarcely find it anywhere now except among the sturgeons, who, we shall see, are old-fashioned too. And now when we inquire into the growth of the shark and the kind of backbone he has, we find that he has still more links with the lower fish-like animals. For when he is young he has nothing but a rod of gristle or cartilage running between the long narrow feeding-tube and the spinal cord; but this rod is flattened in front, and as the young shark grows up the flat part enlarges so as to form a boat-like box--the skull, round the swollen end of the nerve telegraph--the brain. Meanwhile the rod becomes divided into rings, and from each ring an arch of gristle growing upwards surrounds the nerve cord so as to protect it from injury, and the whole skeleton becomes firm and strong. But though the shark is one of the strongest of sea-animals _he never loses this gristly state of his backbone or his skeleton_; however much he may strengthen it by hard matter it never becomes true bone, but remains quite distinct from the skeleton of the bony or osseous fish. [Illustration: Fig. 6. The Sturgeon[13] entering a Russian river.]