512 CHORDATA 



Order VI. Plectognathi. 



Peculiar compact fishes, in which the bones in each jaw are coossified, 

 the upper jaw fused with the cranium, the ventral fins reduced or absent. 

 OSTRACODERMI, the body is enclosed in a firm angular box of bony plates. 

 ( IVMNODONTA, swell fishes (fig. 562), can innate the body to a spherical 

 sac; flesh is poisonous. 



Sub 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 archipterygium. 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 car- 

 tilage, the notochord being retained to a great extent. The pterygoquad- 

 rate fuses with the cranium. The animals live in fresh water and, under 

 ordinary conditions, breathe by gills which are covered by an operculum. 

 Protopterus and the young of Lepidosiren have external as well as internal 

 gills, recalling the Amphibia. The resemblances are strengthened by the 



FIG. 563. Protopterus annectens, lung fish (from Boasj. 



periodic appearance of pulmonary respiration. The lung fishes live in 

 pools and swamps which, during the hot season, may be more or less com- 

 pletely dried up. When the water becomes too foul for branchial respi- 

 ration or dries up the swim bladder is used. This is a paired or unpaired 

 sac with a duct leading to the oesophagus, and has its respiratory surface 

 increased by the development of air cells. Protopterus indeed can live out 

 <>f water; it burrows in the mud at the dry season, and builds a cocoon 

 lined with mucus in which it remains quiescent until the wet season. The 

 nose is respiratory, with choanae opening into the mouth cavity. The 

 last gill vessels give off pulmonary arteries, and there are veins carrying 

 the blood back to the heart, pulmonary and systemic circulations being 

 differentiated. The heart 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. 



