FISHES 



265 



Since the heart receives pure blood from the lung or lungs, 

 as well as impure blood from the general body, the problem of 

 separating the two kinds demands solution; but this is only- 

 effected in an imperfect manner, for there is not even, as in 

 Amphibia (see p. 240), a complete separation of the two auricles 

 which are here present. 



These are the only fishes in which there are internal nostrils 

 as well as external ones. They open just within the margin of 



Fig. 161. — Dipnoi 

 A, Australian Lung- Fish (Cera/(J(^z«), X|. ri, African Lung-Fish (P«7^fl;S^f7-«y), xj. 



the upper lip. The mouth is provided with large dental plates 

 of peculiar form. 



Ceratodus is a large broad fish, attaining the length of 4 or 

 5 feet, and with its body covered by very large scales. The 

 paired fins are broad and paddle-shaped, and each of them is 

 supported by a central axis made up of numerous joints, and of 

 a series of fin-rays attached to the axis in front and behind in 

 a feather-like way. Protopterus is much more slender in form 

 and its paired fins are extremely narrow, the fin-rays on the 

 central axis being largely suppressed. Some specimens are said 

 to reach a length of 6 feet. There are small external gills as 

 well as the gill-folds on the walls of the gill-clefts. Lepidosiren 

 is an eel-like form which may be as much as 6 feet long. Its 



