102 PRIMITIVE ANIMALS 



Ceratodus are exercised in exactly the same way as 

 in Lepidosiren ; the animal coming to the surface 

 of the water to breathe at regular though longer 

 intervals. Ceratodus, however, never burrows in the 

 mud, and during the dry season it continues to live 

 in the ordinary way, seeking out the deeper pools 

 which are not liable to dry up completely. The 

 advantage which Ceratodus obtains from its lungs 

 is that it can subsist in foul water for a much longer 

 period than an ordinary fish, which depends on its 

 gills alone for respiration. 



The earliest known land Vertebrates with typical 

 pentadactyle limbs, the Stegocephali, make their 

 appearance in the lower Carboniferous strata ; the 

 Dipnoi are geologically much older, going back in 

 much their present form to the Devonian and pro- 

 bably still older Palaeozoic formations. Ceratodus, 

 in the identical shape in which it now exists in 

 Australia, is known as a fossil from the Trias of 

 Europe, so that this fish, like the Crustacean Ana- 

 spides, is one of the most interesting of persistent 

 types, with a long geological history, known to us. 

 The extreme antiquity of the Dipnoi favours the view 

 that they are closely related to the true ancestors of 

 the land Vertebrates, and this is further confirmed 

 by resemblances in their anatomy and development 

 which are not clearly associated, like the lungs, with 

 their present habits or mode of life. 



