LUNG-FISHES. 



55 



DIPNOI (Lung-fishes). 



The Dipnoi are termed ' Lung-fishes ' because the existing 

 forms — Ceratodus, Lepidosiren, and Protopterus — have an air- 

 bladder adapted for use as a lung, supplementing and in dry 

 weather supplanting the gills as the organ of respiration. The 

 air-bladder is further comparable with the lung of Amphibians, 

 Reptiles, Birds and Mammals in that it returns the aerated blood 

 direct to the heart, whereas in most fishes the blood from the air- 

 bladder is carried through the general circulation before reaching 

 the heart. 



The body is covered with overlapping cycloid scales. The 



The skull consists of cartilage 



Wall- 

 case 6. 



skeleton is largely cartilaginous. 



ig. : J s2. — Mouth of Ceratodus forsteri, widely open to show the nostrils 

 and teeth, n and n' , narial openings; x, vomerine teeth; xx, palato- 

 pterygoid teeth ; xxx, mandibular teeth. 



covered by membrane bones, i. e. bones which are superficial, 

 developed in the membrane covering the cartilage and not 

 formed by the deposition of salts of lime in the cartilage itself. 

 There are no distinctly differentiated maxillary and premaxillary 

 bones (the bones which form the upper jaw in man), and 

 the functional upper jaw (palato-quadrate cartilage) is confluent 

 with the cranium, a condition designated by the expression ' auto- 

 stylic skull/ a condition also met with in the Chimseras (Wall- 

 case 5). The teeth are few, usually three pairs (fig. 32), and 



