Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/78

58 a sort of slime as a covering for its body, and remaining in this torpid condition, it breathes by means of lungs until the return of the water, when it rises to the surface and breathes by its gills. Hence the Lepidosiren is both Fish and Amphibian. As regards its respiration, it is truly an Amphibian. It differs from the ordinary fish in the structure of its heart, which is composed of three chambers in the Lepidosiren and Amphibian (Siren, Frog, etc.), whereas in the Fishes the heart is composed of only two. The Lepidosiren and Polypterus both have the spiral valve in the intestine, so characteristic of the Sharks. The air-bladder in Polypterus, and the lungs of Lepidosiren, are the same in their structure as regards the arteries of these parts and the relations of their air-ducts. The form of the brain is the same in Lepidosiren and Polypterus. The skull of Lepidosiren is intermediate between the gristly and the bony fishes. The backbone is gristly; in this respect it agrees more with the Fishes than with the Amphibia. In the structure of the liver apparatus and the limbs it agrees with the Amphibia. What is the Lepidosiren? Is it a Fish, or is it an Amphibian? The Lepidosiren is the intermediate form linking the Fishes and Amphibia together, and is more closely allied to the Ganoid Polypterus than any living fish. Among the fossil Ganoids the Coccosteus would represent the Lepidosiren should its skeleton be fossilized. The Ganoid fishes, although intermediate between the Sharks and common bony fishes or Teliosts, have many affinities with the Amphibia: thus, the Amia and Lepidosteus, among the Ganoids, have the air-bladder filled with air-vesicles and resembling strongly the lung of the Amphibia. So the back-bone of the Lepidosteus in the ball-and-socket joint of the pieces forming its spine differs from all Fishes, and agrees with many of the Amphibia. The structure of the Ganoids, Lepidosiren, and Amphibia seems to warrant the conclusion