The young have a bushy external gill, with a broad scaly base. The air-bladder is double, not cellular, with a large air-duct joining the ventral surface of the esophagus. The intes tine has a spiral valve.
The cranium is remarkable for its general ized form, this forming a trait of union be tween the ganoids and the primitive Amphibia or Stegocephali. Without considering terms, tt is not possible to interpret the homol ogies of the cranium of the amphibians and the sharks.
Dipneusti or Lung Dipneusti ( die, twice; irvew, to breathe) are a group characterized by the presence of paired fins consisting of a jointed axis with or without rays. The skull is autostylic, the upper jaw be ing made, as in the Chimera, of palatal ele ments fused with the cranium and without pre maxillary or maxillary. Dentary bones little developed. Air-bladder cellular, used as a lung, in all living species. Heart with many valves in the muscular arterial bulb. Intestine with a spiral valve. Teeth usually of large plates of dentine covered with enamel on the pterygopala tine and splenial bones. Nostrils concealed, when the mouth is closed, under a fold of the upper lip. Scales cycloid, mostly not enameled.
This group has been usually known as Dip noi. But this term was first taken by Leuckart, in 1821. as a name for amphibians, before any of the living Dipneusti were known.
The Ditmelisti agree with the crossoptery gians by the presence of lungs, a character which separates them from all the earlier orders of fishes. 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 prog ress of further divergence. Nothing of the nature of lung or air-bladder is found in lance-• let, lamprey or shark. In none of the remain ing groups of fishes is it wholly wanting at all stages of development.
In the Dipneusti or dipnoans, as in the cros sopterygians and higher vertebrates, the. trachea or air-duct arises from the ventral side of the esophagus. In the more specialized fishiiii.,Tet to be considered, it is transferred to , a side, thus avoiding a turn in passing a the oesophagus itself. From the sharks theli a" • are further distinguished by the p F 'of membrane-bones about the head. P
Actinopteri (ganoids and teleosts) a". ; a•-• and crossopterygians are again the retention of the fringe-fin or a gium as the form of the paired limbs. ' a a. crossopterygians the dipnoans are most readily distinguished by the absence of maxil3 and premaxillary, the characteristic s the jaw of the true fish. The upper jaw m the dipnoan is formed of palatal elements attached directly to the skull, and the lower jaw con tains no true dentary bones. The skull in the dipnoan, as in the Chimera is autost3alic, 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 immovble 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 o ossified skin and not corresponding with the m mbrane-hones of higher fishes. The fish-like opercles, branchiostegals, etc., are not yet dif ferentiated. The teeth have the form of grind ing-plates on the pterygoid areas of the palate, distinctly shark-like in structure. The paired fins are developed as archipterygia, often with out rays, and the pelvic arch consists of a single cartilage, the two sides symmetrical and con nected 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, these scales were long 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 tis sues is essentially as in the Amphibia.