LUNG-FISHES. 



299 



Class YI. — DIPE^OI. 



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Having followed out two lines of the development of 

 the fish-like vertebrates, one leading through the sharks 

 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 

 higher 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 reptiles, at one time being called 

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

 comparatively long time, they remained as members 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 fin-rays. The head is broad and flat, 

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

 mouth. The limbs have a jointed axial skeleton, from 

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

 siren, only the axis remains. Frotopterus occupies a 

 median position in this respect. The notochord persists 

 through life, but the neural and haemal 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 

 groups of fish-like vertebrates. The lungs also are well 

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

 It bears internal pouches, which ponsiderably increase its 

 respiratory surface, and it communicates with the oesopha- 

 gus by a duct which terminates on the ventral surface of 

 the throat, as in the higher vertebrates. In Geratodus 

 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, 

 the lungs, gills persist through life. 



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Pig. 167.— Lung of Geratodus, cut 

 open to show cellular pouches, 

 c; gl, glottis; e, pulmonary 

 vein ; /, arterial blood vessels ; 



03, 



Besides 



