IV. VERTEBRATA: DIPNOI. 579 



Sul Class IV. Dipnoi (Dipneusti). 



The lung fishes have the form of true fishes, with scales and 

 paired fins, supported by a single or a doubly pinnate archiptery- 

 gium. The median fin is not separated into dorsals, caudal and 

 ventral, and the caudal part is diphycercal. The skeleton is very 

 primitive, consisting largely of cartilage, the notochord being re- 

 tained to a great extent. The animals live in fresh water and, 

 under ordinary conditions, breathe by gills which are covered by 

 an operculum. In the gills there are some peculiarities recalling 

 amphibian structures, Protopterus, and the young of Lepidosiren 

 having external as well as internal gills. The resemblances are 

 strengthened by the periodic appearance of pulmonary respira- 

 tion. The lung fishes live in the tropics in pools and swamps 

 which, during the hot season, may be more or less completely dried 

 up. When the water becomes too foul for branchial respiration, 

 the swim bladder is used. This is a paired or unpaired sac with 

 a duct leading to the oesophagus, and the interior has its respira- 

 tory surface increased by the development of air cells. Protopterus 

 indeed can live out of water; it burrows in the mud at the dry 

 season, and builds a cocoon lined with mucus in which it remains 



rifi. oiU. Protojjterus annectens, lung fish. (From Boas.; 



quiescent until the wet season. The nose is respiratory, with a 

 choana opening into the mouth cavity. The last gill vessels give 

 off pulmonary arteries, and there are veins carrying the blood back 

 to the heart. The heart itself shows the beginning of division 

 into arterial and venous halves, especially in the regions of the 

 conus and auricle. 



The few species now living have a wide and discontinuous distribution, 

 and are the remnants of a much richer group which appeared in the 

 paleozoic. MONOPNEUMONIA, with one swim bladder : Ceratodus of Aus^ 

 tralia. D'IPNEUMONIA, with two bladders : Protopterus, Africa ; Lepido- 

 siren, South America. Possibly the paleozoic ARTHRODIRA, some of 

 gigantic size (Dinichthys), belong here. 



