Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/74

56 class of fishes of which the Amphioxus is the representative, then, as Membranous Fishes. The first step in complexity of structure is presented by the simplest of the gristly (cartilaginous) fishes, Lamprey. They possess a gristly skull, with brain, etc., but there is no lower jaw attached to the skull, their mouth being of the sucking kind (Cyclostomi). There are no traces of limbs as yet; but the sucking fishes have a distinct heart, differing from the Amphioxus, wherein we find only slight dilatations of the blood-vessels. The Myxine, or Hag-fish, and the Petromyzon, or Lamprey, are representatives of this order. In the Chimaera we find a lower jaw, but its suspensorium is still immovable. It furnishes the transition from the Lamprey kind to the Sharks. (Fig. 55.) The Sharks and Ray.s (Devil-fish) are still gristly in structure, but their jaws are very freely movable, and furnished with numerous teeth, which are very characteristic in the different kinds. These teeth are found fossil in great numbers in the early rocks, and prove that the gristly fishes were among the first Vertebrates that appeared in the seas. The Sharks possess two pairs of fins, and their intestine is furnished with valves arranged in a spiral or transversely. We come next to a class of fishes known as Ganoids, that is, shining. In some of these, as in the Sturgeon (Fig. 56), we have the backbone still gristly, while in others, as in the Gar-pike, it is bony. The outer part of the body is covered either with shiny plates (Placoganoids), as in the Coccosteus. Sturgeon, or with shiny scales (Lepidoganoids), as in the Gar-pike. It is by means of these shiny plates and scales, as well as the whole fish, found in great profusion, well preserved in the early rocks, that we know that the Ganoids are very old fish, and that they existed in great numbers in the early ages of the earth; whereas at the present day the Ganoids are represented only by half a dozen kinds, the Sturgeon, Gar-pike, Polypterus, etc. The