^246 ZOOLOGY sect. 



number of families, genera, and species, their places being taken 

 by the more highly differentiated Teleostei, until, at the present 

 day, as we have seen, they are reduced to a few scattered forms, 

 mostly confined to fresh waters. 



Sub-class V. The Dipnoi. 



The Dipnoi or Lung-fishes, comprising as their living repre- 

 sentatives only the Queensland CcratoduH or " Burnett Salmon," 

 and the Mud-fishes (ProtojJterus and Lepidosircn) of certain South 

 African and South American rivers, are fishes of such well-marked 

 and special features that by some zoologists they are separated 

 from the true Fishes and regarded as constituting a separate class 

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

 name Dipnoi. Not only do these animals 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, such as is 

 indicated in Polypterus and Amia only among the Teleostomi. 

 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 biserial type 

 (archipterygium, see p. 168). 



1. Example of the Class — Cercdodus (Neoccraiodns) forsteri. 



The Ceratodus or " Burnett Salmon " (Fig. 903) is 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 related genera have been found in abundance in Palaiozoic 

 and Mesozoic beds in Europe, America, the East Indies, Africa, 

 and Australia. Ceratodus forsteri lives in still pools in which the 

 water in the dry season becomes extremel}^ 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 respiration. 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 reiatively large quantities of vegetable matter, 

 which passes with little or no alteration through its enteric canal. 

 Its movements are for the most part very sluggish, and are chiefly 

 effected by the agency of the tail-fin. The paired fins are 

 employed in steering and balancing and in the ascending and 

 descending movements : owing to their groat flexibility they are 

 entirely incapable of supporting the body when the fish is removed 



