230 



ZOOLOGY 



SECT. 



of Vertebrates. One of their peculiar features is indicated by the 



name Dipnoi. Not only do these ani- 

 mals breathe by means of gills, like 

 ordinary Fishes, but they have a highly 

 developed apparatus for the respiration 

 of air a lung or lungs with an 

 arrangement of the circulation co- 

 ordinated with this. They have bony 

 scales and dermal fin-rays, but the 

 paired fins, unlike those of any other 

 fishes, with the exception of certain 

 extinct Elasmobranchs, are constructed 

 on the type of the archipterygium 

 (see p. 155). 



1. EXAMPLE OF THE CLASS Cera- 

 todus Forsteri. 



The Ceratodus or " Burnett Salmon " 

 (Fig. 853) is by far the largest of the 

 Dipnoi, attaining a length sometimes 

 of four or five feet. It occurs at the 

 present day only in the Burnett and 

 Mary Rivers in Queensland, but fossil 

 teeth referred to the same or nearly re- 

 lated genera have been found in abund- 

 ance in Palaeozoic and Mesozoic beds 

 in Europe, America, the East Indies, 

 Africa, and Australia. It lives in still 

 pools in which the water in the dry 

 season becomes extremely stagnant and 

 overladen with decomposing vegetable 

 matter ; and it is only by rising to the 

 surface occasionally, and taking air into 

 its lung, that it is enabled to obtain 

 sufficient oxygen for purposes of re- 

 spiration. Its food consists of such 

 small animals as live among the water- 

 plants and decaying leaves, and in 

 order to obtain a sufficient amount of 

 such food it swallows relatively large 

 quantities of vegetable matter, which 

 passes with little or no alteration 

 through its enteric canal. 



External Characters. The body 

 is fish-like (Fig. 853) with a diphycercal caudal fin. The surface 

 is covered with very large imbricated cycloid scales, somewhat 



