Evolution 



in the Elasmobranchs the skeleton is wholly cartilaginous. One 

 of the most striking anatomical features of these groups, and one 

 which distinguishes them from the Elasmobranchs^ is the presence 

 of a swim bladder, a large sac-like outgrowth from the upper part 

 of the gut. The function of the swim bladder is that of regulating 

 the specific gravity of the fish, which becomes greater or less 

 according as air is expelled or taken in. The Ganoids and bony 

 fishes are illustrated in Figs. 75 to 79. 



The lung fishes or Dipnoi present a curious mixture of primi- 

 tive and of highly advanced characters. In their persistent 

 notochord and their inconsiderable formation of bone, they are 

 much more primitive than the food fishes. On the other hand, 

 an extremely important departure is seen in the adaption of the 



Photo : Underwood. FiG. 75. — Ganold fishes — The Sturgeon. 



swim bladder as a respiratory organ. In one of the three existing 



species this organ is single, in the others it is double. The wall 



of the swim bladder is thick, and contains considerable muscle 



tissue. Its inner surface is covered with a complex system of 



pits and blind sacs, the walls of which contain numerous capillary 



vessels. 



There are three living species of lung fishes, one of which 



is found in Australia, another in Tropical Africa, and a third in 



the tributaries of the Amazon. All live under conditions which 



make ordinary respiration by gills difficult. The Australian 



species inhabits rivers which become reduced, in the dry season, 



to stagnant pools of foul water, in which ordinary fish frequently 



fail to survive. Under such circumstances the creature comes 



periodically to the surface to breathe. The other two species 



live in rivers which actually dry up in summer, and the fishes bury 



98 



