Fig. 258.—A Deep-sea Chimæra, Harriotta raleighiana Goode & Bean. Gulf Stream.
Fig. 259.—An extinct Dipnoan, Dipterus valenciennesi Agassiz. Devonian. (After Pander.)
The Dipnoans.—The most important offshoot of the primitive sharks is not the Chimæras, nor even the shark series itself, but the groups of Crossopterygians and Dipnoans, or lung-fishes, with the long chain of their descendants. With the Dipnoan appears the lung or air-bladder, at first an outgrowth from the ventral side of the œsophagus, as it still is in all higher animals, but later turning over, among fishes, and springing from the dorsal side. At first an arrangement for breathing air, a sort of accessory gill, it becomes the sole organs of respiration in the higher forms, while in the bony fishes its respiratory function is lost altogether. The air-bladder is a degenerate lung. In the Dipnoans the shoulder-girdle moves forward to the skull, and the pectoral limb, a jointed and fringed archipterygium, is its characteristic appendage. The shark-like structure of the mouth remains.
The few living lung-fishes resemble the salamanders in many regards, and some writers have ranged the class as midway between the primitive sharks and the amphibians. These forms show their intermediate characters in the development of lungs and in the primitive character of the pectoral and ventral limbs. Those now extant give but little idea of the great variety of extinct Dipnoans. The living genera are three in number—Neoceratodus in Australian rivers, Lepidosiren in the Amazon, and Protopterus in the Nile. These are all mudfishes, some of them living through most of the dry season encased in a cocoon of dried mud. Of these forms Neoceratodus is certainly the nearest to the ancient forms, but its embryology, owing to the shortening of its growth stages due to its environment, has thrown little light on the question of its ancestry.
From some ally of the Dipnoans the ancestry of the amphibians, and through them that of the reptiles, birds, and mammals may be traced, although a good deal of evidence has been produced in favor of regarding the primitive crossopterygian or fringe fin as the point of divergence. It is not unlikely that the Crossopterygian gave rise to Amphibian and Dipnoan alike.
In the process of development we next reach the characteristic fish mouth in which the upper jaw is formed of maxillary and premaxillary elements distinct from the skull. The upper jaw of the shark is part of the palate, the palate being fused with the quadrate bone which supports the lower jaw. That of the Dipnoan is much the same. The development of a typical fish mouth is the next step in evolution, and with its appearance we note the decline of the air-bladder in size and function.
The Crossopterygians.—The fish-like mouth appears with the group of Crossopterygians, fishes which still retain the old-fashioned type of pectoral and ventral fin, the archipterygium. In the archaic tail, enameled scales, and cartilaginous skeleton the Crossopterygian shows its affinity with its Dipnoan ancestry. Thus these fishes unite in themselves traits of the shark, lung-fish, and Ganoid. The few living Crossopterygians, Polypterus and Erpetoichthys, are not very different from those which prevailed in Devonian times. The larvæ possess external gills with firm base and fringe-like rays, suggesting a resemblance to the pectoral fin itself, which develops from the shoulder-girdle just below it and would seem to give some force to Kerr's contention that the archipterygium is only a modified external gill. In Polypterus the archipterygium has become short and fan-shaped, its axis made of two diverging bones with flat cartilage between. From this type it is thought that the arm of the higher forms has been developed. The bony basis may be the humerus, from which diverge radius and ulna, the carpal bones being formed of the intervening cartilage.
