610 Subclass Dipneusti, or Lung-fishes 



Palaeozoic time, their origin with that or through that of the 

 latter to be traced to the Ichthyotomi or other primitive sharks. 

 These two groups are separated from all the more primitive 

 fish-like vertebrates by the presence of lungs. In its origin 

 the lung or air-bladder arises as a diverticulum from the ali- 

 mentary canal, used by the earliest fishes as a breathing-sac, 

 the respiratory functions lost in the progress of further di- 

 vergence. Nothing of the nature of lung or air-bladder is 

 found in lancelet, lamprey, or shark. In none of the remaining 

 groups of fishes is it wholly wanting at all stages of develop- 

 ment, although often lost in the adult. Among fishes it is most 

 completely functional in the Dipneusti, and it passes through 

 all stages of degeneration and atrophy in the more specialized 

 bony fishes. 



In the Dipneusti, or Dipnoans, as in the Crossopterygians 

 and the higher vertebrates, the trachea, or air-duct, arises, as 

 above stated, from the ventral side of the oesophagus. In the 

 more specialized fishes, yet to be considered, it is transferred 

 to the dorsal side, thus avoiding a turn in passing around the 

 oesophagus itself. From the sharks these forms are further 

 distinguished by the presence of membrane-bones about the 

 head. From the Actinopteri (Ganoids and Teleosts) Dipnoans 

 and Crossopterygians are again distinguished by the presence 

 of the fringe-fin, or archipterygium, as the form of the paired 

 limbs. From the Crossopterygians the Dipnoans are most 

 readily distinguished by the absence of maxillary and pre- 

 maxillary, the characteristic structures of the jaw of the true 

 fish. The upper jaw in the Dipnoan is formed of palatal ele- 

 ments attached directly to the skull, and the lower jaw con- 

 tains no true dentary bones. The skull in the Dipnoans, as 

 in the Chimcera, is autostylic, the mandible articulating directly 

 with the palate 1 apparatus, the front of which forms the upper 

 jaw and of which the pterygoid, hyomandibular and quadrate 

 elements form an immovable part. The shoulder-girdle, as 

 in the shark, is a single cartilage, but it supports a pair of super- 

 ficial membrane-bones. 



In all the Dipnoans the trunk is covered with imbricated 

 cycloid scales and no bony plates, although sometimes the 

 scales are firm and enameled. The head has a roof of well- 



