460 



PISCESSUB-CLASS L— DIPNOI. 



Fig. 1. — Australian Lung-Fish. 



feature distinguishing these fishes from all other members of the class. , The 

 small and compressed gills open externally by a single aperture which is pro- 

 tected by an operculum, from beneath which may protrude stunted and 



rudimental external gills. The 

 nerves supplying the eyes, where 

 they cross one another, have an 

 interlacing of their fibres, and the 

 short digestive tract exhibits the 

 shark-like feature of a spiral valve 

 in the membrane of the intestine. 

 Not only is the elongated and 

 sacculated air-bladder provided with 

 a duct, but it acts in all respects as 

 a lung, with which indeed it is in 

 every sense comparable. The 

 backbone is mainly notochordal, 

 true vertebrae being developed only 

 in the caudal region. The denti- 

 tion is of a very peculiar type, and consists of a few very large and 

 strongly-ridged plates, most of which are paired, and divided by a distinct 

 median line. 



Lung-fishes are now represented only by three species, belonging to as 

 many distinct gsnera, but all included in the single family LepidodrenidcB. 

 Of these, the least specialised is the Australian lung-fish (Ceratodus), which in 

 the form of its paired fins retains more of the primitive type than does either 

 of the other two. The scales are very large, and the paired fins are broad 

 and lobate, consisting of a central scaled portion bordered by a fringe. Mar- 

 ginal teeth are wanting in the jaws ; but the palate contains a pair of large 

 dental plates behind, and u. couple of smaller chisel-like teeth in front, the 

 lower jaw having a single pair of large plates. Long before it was known to 

 science, the Australian lung-fish was familiar to the settlers on the banks of 

 the two great rivers of Queensland, on account of the excellent quality of its 

 salmon-coloured flesh ; — whence the name of Burnett salmon commonly 

 applied to it. The fish, which grows to a couple of yards in length, lives 

 among mud and leaves, and rises at 

 times to the surface to breathe. 

 Its food consists of both vegetable 

 and animal substances ; and the 

 large eggs are deposited singly. In 

 the two remaining forms — one of 

 which (Lepidusiren) inhabits the 

 rivers of Tropical South America, 

 and the other (Prutoptertis) those 

 of Africa — the body is more eel- 

 like, the limbs are reduced to whip- 

 like filaments, and the anterior teeth 

 are situated in the margins of the 

 jaws. In many parts of the country 

 the African lung-fish during the 

 dry season envelops itself in a kind 

 of shell or cocoon of mud, and in this state it can be dug up and exported, 

 reviving when placed in warm water. Dr. Dean observes that "lung-fishes 



Fig. 2.— African Lukq-Fish. 



