The Dipnoi

The "Mudfishes" of the rivers of the east and west coasts of Africa and of eastern South America are nearly transitional forms between the Pisces and the Amphibia.

The eel-like body, covered with overlapping cycloid scales, tapers to a point at its caudal extremity, and is provided with two pairs of long, ribbon-like, pointed extremities, and with a caudal fin.

The spinal column consists of a thick notochord, invested by a cartilaginous sheath, without any osscous or cartilaginous vertebral centra. The proximal ends of ossified neural arches, of ribs, and, in the caudal region, of inferior arches, are imbedded in the sheath of the notochord.

Fin-rays support the median fin. The skull, the palato-quadrate, and suspensorial apparatus, form, as in Chimaera, one continuous cartilaginous mass, into the base of which the notochord penetrates, terminating in a point behind the pituitary fossa.

No cartilage bone is developed in the place of the basi-occipital, supra-occipital basisphenoid, or presphenoid; and there are only two such ossifications, which represent the ex-occipitals (E. 0.) in the side-walls of the cranium. A large parasphenoid (x) underlies the base of the skull. Upon its roof a great single bone (A), answering to the parietals and frontals, extends from the occipital to the ethmoidal regions. In front of this are two nasal bones (C). There is no alisphenoid, but the fronto-parietal and parasphenoid send processes toward one another, which unite in front of the exit of the third division of the fifth nerve. There is no interorbital septum, and the cavity of the skull remains of tolerably even diameter throughout. In front of the exit of the optic nerves, however, it is longitudinally divided by a membranous septum.

The ethmovomerine cartilage is continued to the anterior extremity of the skull. It bears teeth, but no distinct vomer.

A great palato-pterygoid osseous arch (D) extends from the middle line along the upper and the under surface of the palato-quadrate arch on each side to near the articular surface of the mandible. In the middle of the roof of the mouth, divergent, cutting, dentary plates are developed upon it. An osseous nodule lies in the articular head of the palato-quadrate cartilage, and is continuous with the bone F.

The mandible presents dentary plates corresponding with those of the palate, and biting between the latter. The hyoidean arch is attached to the posterior and lower edge of the suspensorium - which bears a bony ray representing an operculum - while the hyoidean arch itself carries a single branchiostegal ray (Br, Fig. 52).

The pectoral arch is composed of a median cartilaginous part, with two lateral portions of cartilage, at once separated from, and connected with, the median cartilage by bone. The bone is separated from the cartilage by a layer of connective tissue, and seems to represent the clavicle, while the cartilage answers to the coalescent coraco-scapular cartilages of other fishes.

The filiform fin is supported by a many-jointed cartilaginous rod, articulated proximally with the coraco-scapular. Upon this are disposed fine fin-rays like those of the Elasmobranchs, which support the marginal fringe of the fin. The ventral fin has the same structure as the pectoral.

The intestine possesses a spiral valve, and the rectum opens into a cloaca. The lungs have remarkably stiff walls, and extend through the greater part of the body, beneath the spine. The glottis, opening upon the ventral wall of the gullet, places them in communication with the cavity of the mouth, into which the nasal sacs open by posterior apertures, which lie inside the upper lip and constitute true posterior nares. The heart has a small, but distinct, left auricle, into which the blood which has been aerated in the lungs is returned. In addition to lungs, Lepidosiren possesses both internal and external gills, but the latter are rudimentary in the adult.

The different species seem to differ in the manner in which the primitive aortic arches are metamorphosed; but it may be said, generally, that the first has disappeared; the second supplies an internal branchia developed upon the hyoidean arch; the third gives off the anterior carotid artery, and supplies neither internal nor external branchia; the fourth supplies only the first external branchia; the fifth and sixth supply both internal and external branchiae; while the seventh is connected only with an internal branchia. The pulmonary artery seems originally to have been given off from an eighth aortic arch.

It is a remarkable circumstance that, while the Dipnoi present, in so many respects, a transition between the piscine and the amphibian types of structure, the spinal column and the limbs should be not only piscine, but more nearly related to those of the most ancient Crossopterygian Ganoids than to those of any other fishes.