There remains the fourth subclass—Dipnoi or Dipneusti, the lungfishes. The reason why these creatures, whose organization is on an antique and lowly plane, judged by fish standards, have been elevated to subclass rank is that here the air bladder is modified into a single or double elongated sac with many cellular spaces, and is connected by a short tube with the mouth, and thus serves as a lung. The peculiar structure of the heart, narial openings, and the power of existing for a considerable period out of water, are extremely amphibianlike, and they have by various naturalists been regarded as scaly sirens—a sort of connecting link between the fishes and the amphibians. They are found fossil in Paleozoic rocks, especially in the Old Red Sandstone of Great Britain, and also in the Upper Jurassic strata in Colorado.
| AN AFRICAN LUNGFISH |
| (Protopterus annectens) |
The surviving species (family Lepidosirenidæ) are widely scattered, as is characteristic of all these very ancient families. A celebrated example is the barramundi of Queensland—an elongated, flat-sided fish, covered, except on the head, with large roundish scales, and having paired fins that look more like flippers than fins. It becomes four or five feet long. It lives in still pools in which the water in the dry season becomes extremely stagnant and overladen with decomposing vegetable matter; and it is only by rising to the surface occasionally, and taking air into its lung, that it is enabled to obtain sufficient oxygen for purposes of respiration. The barramundi does not leave the water, nor can it live long in the air. It is easily captured, and is eaten by the blackfellows.
Equatorial Africa possesses three species of the genus Protopterus, which dwell in marshes, and feed voraciously on young fishes, frogs, and small animals. The form is somewhat eellike, and the paired fins are soft, slender appendages of little use, locomotion being effected by the powerful tail. Like the barramundi this fish rises at intervals to take a breath of air; its "lungs" are double, while that of the barramundi is single. In the dry time of summer the protopterus burrows deeply into the mud of the dried-up marshes, where it curls up with its head highest and subsists wholly by breathing air until the autumnal rains bring water enough to enable it to wake up and resume its aquatic life. A similar eellike species abounds in the swamps, sluggish rivers and marshes of northern South America, named Lepidosiren, and all its habits closely resemble those of the African lungfishes.
[CHAPTER XVIII]
AMPHIBIANS—A CONNECTING LINK
We have now come to a class of vertebrates that in their manner of life, and presumably in their history, connect the dwellers in the waters with those on the lands of the globe. Dr. Gamble cites examples from various groups of animals to show that adaptation to a terrestrial existence is an advance on that requisite for aquatic life, and that the critical point in the evolution of the vertebrate phylum was passed when its members migrated from water to land. "When we come to land animals," he says, "the problem of weight has to be considered before that of locomotion. The lateral undulations of the body, even when aided by unjointed paddles, or fins, are not sufficient to insure rapid movement on land. Hence a system of levers has to be evolved, partly to support the body, and partly to propel it. The use of joints becomes a necessity, and we find that all active terrestrial animals, except snakes, have jointed limbs. The critical point in the history of this phylum is passed when its members migrated from water to the land. The step was taken by the ancestors of the Amphibia (that is, the frogs, toads, and salamanders). In them the breast fins of the fish have become the jointed forelegs, the pelvic fins have become the hind legs."
How this great change from the fish fin to the five-fingered hand occurred is, at present, just as obscure as the mode of conversion of the arms of reptiles into the wings of birds. The answer can only be supplied by further discoveries in the geological history of the order, and though this history can be traced back to the time of the Coal Measures, we find the earliest Amphibia as sharply marked off from the fishes by their feet as they are to-day. These forefathers (subclass Stegocephalia) are the earliest known four-footed animals, and their fossil skeletons are found from the Carboniferous up to the Trias, after which the race disappears. They had the general form of newts, and many were only a few inches in length. That some of these, at least, were terrestrial in habit is shown by the fact that they are often found in stumps and hollow logs of sigillarias and other fossil trees of the coal beds, especially in Nova Scotia. But there were also species several feet in length, with formidable teeth, which were no doubt carnivorous and predatory, so that it was well for the little ones to seek places of safety. These stegocephalians were unmistakably amphibians, with two condyles supporting the skull, but their skeleton contains many features that suggest reptilian anatomy, and it is agreed that the reptiles sprang from this stock. The peculiar feature of this group is that their flattish heads were covered by a broad shield of bony plates (ossified skin); and similar armor protected their bellies, and in a few cases the back also.