A Guide to the Study of Fishes, Volume 1 (of 2)

by David Starr Jordan

Fig. 381.—Shoulder-girdle of Neoceratodus forsteri Günther. (After Zittel.)

The lung-fishes, or Dipneusti (δίς, two; πνεῖν, to breathe), arise, with the Crossopterygians, from the vast darkness of Palæozoic time, their origin with that or through that of the latter to be traced to the Ichthyotomi or other primitive sharks. These two groups are separated from all the more primitive fish-like vertebrates by the presence of lungs. In its origin the lung or air-bladder arises as a diverticulum from the alimentary canal, used by the earliest fishes as a breathing-sac, the respiratory functions lost in the progress of further divergence. Nothing of the nature of lung or air-bladder is found in lancelet, lamprey, or shark. In none of the remaining groups of fishes is it wholly wanting at all stages of development, although often lost in the adult. Among fishes it is most completely functional in the Dipneusti, and it passes through all stages of degeneration and atrophy in the more specialized bony fishes.

In the Dipneusti, or Dipnoans, as in the Crossopterygians and the higher vertebrates, the trachea, or air-duct, arises, as above stated, from the ventral side of the œsophagus. In the more specialized fishes, yet to be considered, it is transferred to the dorsal side, thus avoiding a turn in passing around the œsophagus itself. From the sharks these forms are further distinguished by the presence of membrane-bones about the head. From the Actinopteri (Ganoids and Teleosts) Dipnoans and Crossopterygians are again distinguished by the presence of the fringe-fin, or archipterygium, as the form of the paired limbs. From the Crossopterygians the Dipnoans are most readily distinguished by the absence of maxillary and premaxillary, the characteristic structures of the jaw of the true fish. The upper jaw in the Dipnoan is formed of palatal elements attached directly to the skull, and the lower jaw contains no true dentary bones. The skull in the Dipnoans, as in the Chimæra, is autostylic, the mandible articulating directly with the palatal apparatus, the front of which forms the upper jaw and of which the pterygoid, hyomandibular and quadrate elements form an immovable part. The shoulder-girdle, as in the shark, is a single cartilage, but it supports a pair of superficial membrane-bones.

In all the Dipnoans the trunk is covered with imbricated cycloid scales and no bony plates, although sometimes the scales are firm and enameled. The head has a roof of well-developed bony plates made of ossified skin and not corresponding with the membrane-bones of higher fishes. The fish-like membrane-bones, opercles, branchiostegals, etc., are not yet differentiated. The teeth have the form of grinding-plates on the pterygoid areas of the palate, being distinctly shark-like in structure. The paired fins are developed as archipterygia, often without rays, and the pelvic arch consists of a single cartilage, the two sides symmetrical and connected in front. There is but one external gill-opening leading to the gill-arches, which, as in ordinary fishes, are fringe-like, attached at one end. In the young, as with the embryo shark, there is a bushy external gill, which looks not unlike the archipterygium pectoral fin itself, although its rays are of different texture. In early forms, as in the Ganoids, the scales were bony and enameled, but in some recent forms deep sunken in the skin. The claspers have disappeared, the nostrils, as in the frog, open into the pharynx, the heart is three-chambered, the arterial bulb with many valves, and the cellular structure of the skin and of other tissues is essentially as in the Amphibian.

The developed lung, fitted for breathing air, which seems the most important of all these characters, can, of course, be traced only in the recent forms, although its existence in all others can be safely predicated. Besides the development of the lung we may notice the gradual forward movement of the shoulder-girdle, which in most of the Teleostomous fishes is attached to the head. In bony fishes generally there is no distinct neck, as the post-temporal, the highest bone of the shoulder-girdle, is articulated directly with the skull. In some specialized forms (Balistes, Tetraodon) it is even immovably fused with it. In a few groups (Apodes, Opisthomi, Heteromi, etc.) this connection ancestrally possessed is lost through atrophy and the slipping backward of the shoulder-girdle leaves again a distinct neck. In the Amphibians and all higher vertebrates the shoulder-girdle is distinct from the skull, and the possession of a flexible neck is an important feature of their structure. In all these higher forms the posterior limbs remain abdominal, as in the sharks and the primitive and soft-rayed fishes generally. In these the pelvis or pelvic elements are attached toward the middle of the body, giving a distinct back as well as neck. In the spiny-rayed fishes the "back" as well as the neck disappears, the pelvic elements being attached to the shoulder-girdle, and in a few extreme forms (as Ophidion) the pelvis is fastened at the chin.

Classification of Dipnoans.—By Woodward the Dipneusti are divided into two classes, the Sirenoidei and the Arthrodira. We follow Dean in regarding the latter as representative of a distinct class, leaving the Sirenoidei, with the Ctenodipterini, to constitute the subclass of Dipneusti. The Sirenoidei are divided by Gill into two orders, the Monopneumona, with one lung, and the Diplopneumona, with the lung divided. To the latter order the Lepidosirenidæ belong. To the former the Ceratodontidæ, and presumably the extinct families also belong, although nothing is known of their lung structures. Zittel and Hay adopt the names of Ctenodipterini and Sirenoidei for these orders, the former being further characterized by the very fine fin-rays, more numerous than their supports.

Order Ctenodipterini.—In this order the cranial roof-bones are small and numerous, and the rays of the median fins are very slender, much more numerous than their supports, which are inserted directly on the vertebral arches.

In the Uronemidæ the upper dentition comprises a cluster of small, blunt, conical denticles on the palatine bones; the lower dentition consists of similar denticles on the splenial bone. The vertical fins are continuous and the tail diphycercal. There is a jugular plate, as in Amia. The few species are found in the Carboniferous, Uronemus lobatus being the best-known species.

In Dipteridæ there is a pair of dental plates on the palatines, and an opposing pair on the splenials below. Jugular plates are present, and the tail is usually distinctly heterocercal.

In Phaneropleuron there is a distinct anal fin shorter than the very long dorsal; Phaneropleuron andersoni is known from Scotland, and Scaumenacia curta is found at Scaumenac Bay in the Upper Devonian of Canada.