Mud Fishes. 



33 



the most singularly shaped of FlG - x 



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 i 

 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 

 Lcpidosiren (fig. 14), of South 

 America ; and the third is the (The mud fish, Lepidosiren\ 



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