RESPIRATION 51 



forms, and to a more marked extent in the Lung-fishes, the 

 inner walls are richly supplied with blood-vessels, and the area 

 of their surface is greatly increased by being produced into 

 recesses or alveoli, each of which is further subdivided much 

 in the same way as in a true lung (Fig. 22). Another important 

 point is the fact that, whereas in nearly all fishes the bladder is 

 a single structure, in the Bow-fin, Gar Pike, Bichir and Lung- 

 fishes it tends to become divided, giving rise to a structure 

 resembhng the paired lungs of higher vertebrates. In the 

 Bichirs, for example, it is divided into two unequal parts, a 

 long right-hand portion and a much shorter left-hand one; 

 the two unite in front and open by a single aperture in the 

 floor of the gullet. The air-bladder of the African (Protopterus) 

 and South American [Lepidosiren) Lung-fishes is divided into 

 two except for a small portion in front, and thus has the form 

 of a pair of lungs. Finally, it may be noticed that the arrange- 

 ment of the vessels taking the blood to and from the bladder 

 in the Bichirs and Lung-fishes is essentially similar to that of 

 the vessels connected with the lungs in Amphibians and 

 reptiles. 



There is good reason for supposing that the air-bladder 

 originally had a purely respiratory function, and that its use 

 as a hydrostatic organ is secondary. The essential similarity 

 of the mode of development of the air-bladder and the lungs 

 is a striking argument in favour of this view; and further, the 

 bladder is connected with the gullet by a pneumatic duct 

 during the early embryonic or larval stages of most, if not all, 

 Bony Fishes, and this connection is retained throughout life in 

 nearly all the more primitive forms. The air-breathing fishes 

 of to-day, such as the Lung-fishes, Bichirs, Bow-fins and Gar 

 Pikes, are without exception survivors of very ancient groups 

 which flourished at a very early stage of geological time, 

 whereas, all those in which the air-bladder serves as a hydro- 

 static organ are of comparatively modern origin. We may 

 note here that no trace of an air-bladder is found in any 

 Selachian, nor is one developed in the Lampreys and their 

 allies. The statement so often made that the lungs of higher 

 animals have developed from the air-bladder of fishes is quite 

 untrue, and this idea rests on a misconception similar to that 

 which leads many people to speak of man having descended 

 from monkeys. It would be more correct to speak of the air- 

 bladder as a modified and degenerate lung, but actually both 

 bladder and lung seem to have been derived from some sort of 



