LUNG-FISHES. 



299 



Class VI. — DIPNOI. 



Having followed out two lines of the development of 

 the fish-like vertebrates, one leading through the sliarks 

 and skates, the other through the true bony fishes, we 

 return again to the central stem, and take up the lung- 

 fishes, the Dipnoi of scientific terminology, which appar- 

 ently lead from the ganoids to the Batrachia and the 

 liigher vertebrates. For many years but two living gen- 

 era belonging to this class were known, and these, from 

 their curious combination of characters, were bandied 

 about between fishes and re]itiles, at one time being called 

 fish-like reptiles, and at others reptilian fish. Then, for a 

 comparatively long time, they remained as membere of the 

 ganoids, with which they have many undoubted affinities. 

 Exactly where they will ultimately be placed cannot be 

 told until their embryology is studied. Of this we as yet 

 know absolutely nothing. During this uncertainty it seems 

 best to place them here, next the amphibians, between 

 which and the ganoids they are the representatives of 

 the connecting links now long extinct. 



In external appearance these forms are much like the 

 ganoids. The body is long and eel-like, and, like the head, 

 is covered by scales, and terminates in a compressed cau- 

 dal fin, with weak fiu-rays. The head is broad and flat, 

 and the two pairs of nostrils are more or less within tlie 

 mouth. The limbs have a jointed axial skeleton, from 

 which, in Ceratodus, jointed rays diverge, but in Lepido- 

 siren, only the axis remains. Protopterus occupies a 

 median position in this respect. The notochord pereists 

 through life, but the neural and hremal arches are ossified. 

 The heart is three-parted, and is composed of one ventricle 

 and two auricles, the second and smaller auricle receiving 

 the blood as it returns from the lungs. This structure 

 is different from that occurring in any of the preceding 

 grou])s of fish-like vertebrates. The lungs also are well 

 developed, as is indicated liy the popular name, lung-fishes. 

 It bears internal pouches, which considerably increase its 

 respiratory surface, and it communicates with the a?sopha- 

 gus by a duct which terminates on the ventral surface of 

 the throat, as in the higher vertebrates. In Ceratodus 

 there is but one lung, but in this we observe a symmetri- 

 cal arrangement into right and left halves, while in the 

 other genera the organ is completely divided into right and left lungs. Besides 

 the lungs, gills ])ersist through life. 



Fig. 167.— Lung of Cmloilus, cut 

 open to show cellular pouches, 

 c; gl, glottis ; e, puhiionary 

 vein": /', .arterial blood vessels ; 

 (e, (esophagus. 



