We may divide the Teleostomes, or true fishes, into three subclasses: the Crossopterygii, or fringe-fins; the Dipneusti, or lung-fishes; Actinopteri, or ray-fins, including the Ganoidei and the Teleostei, or bony fishes. Of these many recent writers are disposed to consider the Crossopterygii as most primitive, and to derive from it by separate lines each of the remaining subclasses, as well as the higher vertebrates. The Ganoidei and Teleostei (constituting the Actinopteri) are very closely related, the ancient group passing by almost imperceptible degrees into the modern group of bony fishes.
Subclass Crossopterygii.—The earliest Teleostomes known belong to the subclass or group called after Huxley, Crossopterygii (κρόσσος, fringe; πτερύξ, fin). A prominent character of the group lies in the retention of the jointed pectoral fin or archipterygium, its axis fringed by a series of soft rays. This character it shares with the Ichthyotomi among sharks, and with the Dipneusti. From the latter it differs in the hyostylic cranium, the lower jaw being suspended from the hyomandibular, and by the presence of distinct premaxillary and maxillary elements in the upper jaw. In these characters it agrees with the ordinary fishes. In the living Crossopterygians the air-bladder is lung-like, attached by a duct to the ventral side of the œsophagus. The lung-sac, though specialized in structure, is simple, not cellular as in the Dipnoans. The skeleton is more or less perfectly ossified. Outside the cartilaginous skull is a bony coat of mail. The skin is covered with firm scales or bony plates, the tail is diphycercal, straight, and ending in a point, the shoulder-girdle attached to the cranium is cartilaginous but overlaid with bony plates, and the branchiostegals are represented by a pair of gular plates.
In the single family represented among living fishes the heart has a muscular arterial bulb with many series of valves on its inner edge, and the large air-bladder is divided into two lobes, having the functions of a lung, though not cellular as in the lung-fishes.
The fossil types are very closely allied to the lung-fishes, and the two groups have no doubt a common origin in Silurian times. It is now usually considered that the Crossopterygian is more primitive than the lung-fish, though at the same time more nearly related to the Ganoids, and through them to the ordinary fishes.
Origin of Amphibians.—From the primitive Crossopterygii the step to the ancestral Amphibia, which are likewise mailed and semi-aquatic, seems a very short one. It is true that most writers until recently have regarded certain Dipneustans as the Dipteridæ as representing the parents of the Amphibians. But the weight of recent authority, Gill, Pollard, Boulenger, Dollo, and others, seems to place the point of separation of the higher vertebrates with the Crossopterygians, and to regard the lobate pectoral member of Polypterus as a possible source of the five-fingered arm of the frog. This view is still, however, extremely hypothetical and there is still much to be said in favor of the theory of the origin of Amphibia from Dipnoans and in favor of the view that the Dipnoans are also ancestors of the Crossopterygians.
Fig. 370.—Shoulder-girdle of Polypterus bichir. Specimen from the White Nile.
In the true Amphibians the lungs are better developed than in the Crossopterygian or Dipnoan, although the lungs are finally lost in certain salamanders which breathe through epithelial cells. The gills lose, among the Amphibia, their primitive importance, although in Proteus anguineus of Austria and Necturus maculosus, the American "mud-puppy" or water-dog, these persist through life. The archipterygium, or primitive fin, gives place to the chiropterygium, or fingered arm. In this the basal segment of the archipterygium gives place to the humerus, the diverging segments seen in the most specialized type of archipterygium (Polypterus) become perhaps radius and ulna, the intermediate quadrate mass of cartilage possibly becoming carpal bones, and from these spring the joints called metacarpals and phalanges. In the Amphibians and all higher forms the shoulder-girdle retains its primitive insertion at a distance from the head, and the posterior limbs remain abdominal.
The Amphibians are therefore primarily fishes with fingers and toes instead of the fringe-fins of their ancestors. Their relations are really with the fishes, as indicated by Huxley, who unites the amphibians and fishes in a primary group, Ichthyopsida, while reptiles and birds form the contrasting group of Sauropsida.
