Mud Fishes. 



33 



the most singularly shaped of 

 tropical fishes. One remarkable 

 species the sun fish a large 

 globular fish with an extraordi- 

 narily thick skin, sometimes 

 reaches the weight of 400 

 pounds. 



1 8. Order 5, Dipnoi.- This, 

 the last and in some respects 

 most interesting order, includes 

 three living fishes, which form 

 a transition to the next class. 

 These fishes differ from all the 

 foregoing in having the swim- 

 ming-bladder developed as an 

 accessory respiratory organ ; the 

 blood returning from it being 

 received into a small additional 

 auricle of the heart placed to 

 the left of the main auricle. 

 They have a covering of horny 

 scales, and the alimentary canal 

 has a spiral valve. They also 

 exhibit the peculiarity of pos- 

 sessing tubular nasal passages 

 which perforate the upper lip, 

 opening into the mouth. One 

 of the fishes of this order is the 

 African mud fish, or Protopterus 

 of the Gambia ; another is the 

 Lepidosiren (fig. 14), of South 

 America ; and the third is the (The mud fish 



D 



