EXTANT MUD-FISHES. 119 



former. Their external appearance is entirely like that of 

 Fishes. 



The head of the Dipneusta is not distinct from the 

 trunk. The skin is covered with large fish-scales. The 

 skeleton is soft, cartilaginous; its development has been 

 arrested at a very low stage, just as in the lower Primitive 

 Fishes. The notochord is retained entire. The two pairs of 

 limbs are very simple fins of primitive structure, like those 

 of the lowest Primitive Fishes. The structure of the brain, 

 of the intestinal tube, and the sexual organs, is also as in 

 Primitive Fishes. The Dipneusta, or Mud-fishes, have, there- 

 fore, by heredity, accurately retained many features of a 

 lower organization derived from our primaeval Fish ancestors, 

 while their adoption of the habit of breathing air through 

 lungs introduced a great advance in the vertebrate organi- 

 zation. 



Moreover, the three extant Mud-fishes differ a good deal 

 from one another in important points of structure. The 

 Australian Mud-fish (Ceratodus), which was first described 

 at Sidney in 1870 by Gerard Krefit, and which attains a 

 length of six feet, appears in an especial degree to represent 

 a primaeval and very conservative animal form (Plate XII.). 

 This is especially true of the structure of its simple lung, 

 and of its fins, which contain a pinnate skeleton. In the 

 African Mud-fish (Protopterus), on the contrary, and in the 

 American form (Lepidosireii) the double lung is present, as 

 in all higher Vertebrates ; nor is the fin-skeleton pinnate. 

 In addition to the internal gills, Protopterus has also ex- 

 ternal gills, which are wanting in Lepidosiren. Those 

 unknown Dipneusta, which were among our direct ancestors, 

 and which formed the connecting link between the Selachii 



