. Natural history. Zoology. 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
. Natural history. Zoology. 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 su
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