ANCIENT ANIMALS 429 



Air-breathing fishes. Remarkable as it may seem, all of the Devonian 

 bony fishes possessed lungs in addition to gills and could breathe air. 

 This can be inferred from the comparative anatomy of the fossil fishes 

 and their descendants and is made certain by a fossil in which the out- 

 lines of the lung have been preserved. In the ray fins the primitive lung 

 became transformed into the air bladder which enables them to float at 

 any desired level in the water; but in the choanate fishes and their terres- 

 trial descendants it retained its air-breathing function. The lung of the 



Fig. 27.12. The primitive Devonian shark, Cladoselache. {Redrawn from Romer, by permis- 

 sion The Williams & Wilkins Company.) 



Fig. 27.13. The primitive Devonian ray-finned fish, Palaeoniscus. (Redrawn from Romer, 

 The Vertebrate Body, by permission W. B. Saunders Company.) 



Devonian fishes was probably an adaptation to life in ponds and streams 

 which in dry seasons were reduced to stagnant pools, where ordinary 

 fishes would die but those with lungs might survive until the next rains. 

 The nature of the Devonian sediments supports this interpretation, for 

 they are of a type which is formed under conditions of alternate sub- 

 mergence and exposure to air. The hypothesis is further strengthened by 

 the habits of the five genera of air-breathing fishes which survive today 

 as veritable living fossils. Two of these are primitive ray fins inhabiting 

 African rivers; the others are hingfisb.es occurring in Australia, Africa, 

 and South America. 



The Australian lungfish lives in pools and water holes, cropping the aquatic 

 vegetation to obtain the snails, insects, and other small animals found thereon. 



