VI 



THE LUNG-FISHES 



Lung-fishes, or Dipnoans, have long been looked upon 

 as the linking type between amphibians and fishes. In 

 some regards of structure they approach the primitive 

 sharks ; in others, they resemble so closely the salamanders 

 that they were recently regarded by W. N. Parker as worthy 

 of a class by themselves, intermediate between fishes and 

 amphibians. As with the Chimaeroids, their few surviving 

 members give but a mere suggestion of the former size 

 and importance of the group. 



Structural Characters 



The general structural plan of a Dipnoan is shown in 

 the adjoining figure (Fig. 121), taken from a dissection of 

 the African form, Protopterus. Its thick, spindle-shaped 

 body, enclosed in rounded, horn-like scales, CS, terminates 

 in a diphycercal tail, CF. The head is salamander-like 

 both in shape and in slimy integument. The paired fins 

 (schematized in the figure, PF, VF) are archipterygial. 



The head region is characterized by a cartilaginous brain 



case, roofed by dermal bones, HR ; a mandible, MA, 



directly articulated with the skull (autostylic) ; an anterior 



and posterior nares, NO, — the former opening under the 



lip, the latter within the mouth ; a row of small, compressed 



(un segmented) gill arches, GA, whose single outer aperture 



116 



