LUX a FISHES. 



429 



many-jointed cartilaginous rod, to which is attached fine fin- 

 rays, supporting the thin edge of the fin. 



The spiral valve and cloaca are present in the intestinal 

 tract. In Polypterus the cerebral hemispheres are larger 

 than the olfactory lobes, and there is an optic chiasma ; the 

 heart has besides the right large auricle, a left smaller one 

 which receives the blood from the lungs, and a single ven- 

 tricle, as in Amphibians and most reptiles ; they have true 

 nostrils. The lungs are like those of Amphibians, and in 

 addition they possess both 

 internal and external gills 

 (Fig. 393), the latter nearly 

 aborted in the adult. 



The genus Ceratodus was 

 originally named by Agassiz, . F js- m External gills of a young PO- 



J / & . lypterus bichir. Jr, external gills. 



from teeth found in Jurassic 



and Triassic strata in Europe. Living specimens were found 

 by Mr. Krefft in Queensland, Australia, and called Ceratodus 

 Fosteri Krefft (Fig. 394). This fish is rather more elemen- 

 tary in form than Lepidosiren, the body being stouter, and 

 the large scales of the body, with the fin-like paddles and 

 distinctly rayed vertical fins, cause it to resemble more closely 

 ordinary bony fishes than Lepidosireii (Griinther). Moreover 



Fig. 394. Ceratodus, or Australian Lung-Fish. (The tail in nature ends in a point.) 

 After Giinther ; from Nicholson. 



the lung is single, and not used so much as the two perfect 

 lungs of Lepidosiren. It attains a length of six feet. It 

 can breathe by either gills or lungs alone. When, Gtinther 

 thinks, the fish is compelled to live during droughts in thick 

 muddy water charged with gases which are the product of 

 decomposing organic matter, it is obliged to use its lungs. 

 The gills are more like those of ordinary bony fishes than 

 those of Lepidosiren. It lives on the dead leaves of aquatic 

 etc. The local English name is "flat-head," the 



