340 COMPARATIVE ZOOLOGY OF CHORDATES 



In the primitive form Amia, it is still supplied with blood from 

 the last branchial artery and its walls are cellular, but in all 

 the rest it derives blood from the coeliac artery and dorsal 

 aorta, and its walls are not adapted for the diffusion of gases 

 through them, except in a restricted vascular area. In some 

 forms it remains connected with the alimentary canal by an 

 open tube, but in others it is completely shut off (in the adult 

 condition). In these higher bony fish, the lung no longer 

 functions as a respiratory organ, but it has become a hydro- 

 static organ. The quantity of gas which it contains is regulated 

 by the vascular area just referred to (where oxygen may be 

 passed from the blood into it or vice versa), and the fish is able 

 to adapt its specific gravity to that of the depth of the water 

 at which it is swimming. It is therefore able to maintain 

 its depth without muscular effort. In these forms it is no 

 longer a lung, but an air-bladder or swim-bladder. In some 

 Teleosts, such as the catfish (Amiurus), the swim-bladder 

 enters into relations with the auditory vesicle, and is connected 

 with it by a chain of small bones called the Weberian ossicles, 

 which are derived from the first three vertebrae. In some other 

 Teleosts, the swim-bladder disappears in the adult, and these 

 are often found to be bottom-living forms, which live at a more 

 or less constant depth. 



Strange as it may seem, therefore, it is probable that the 

 lungs were evolved while the vertebrates were still in the 

 water, and that they gave rise to the swim-bladder by 

 specialisation. 



It is now necessary to turn to the relations which the 

 olfactory organs bear to the respiratory system. In the 

 Selachii and the higher bony fish, the nasal sacs have no 

 connexion with the mouth, but this is not the case in the most 

 primitive bony fish. In Osteolepis and in the Dipnoi there 

 are external nostrils on the snout, and they lead to internal 

 nostrils which open into the mouth-cavity. This condition 

 is also present in all the Tetrapods. In these forms, therefore, 

 the olfactory organs are subservient to the respiratory system 

 in that they enable the respiratory medium (water or air) to 

 enter the mouth- cavity without having to pass through the 



