Ill 



AIK-BLADDEE 



173 



situated laterally, balancing one. another, while farther back where 

 only the right lung is present this shifts towards the mesial plane 

 until it is symmetrical about that plane, lying in the dorsal 

 mesentery (Fig. 98, A and B). 



Evolution of the Air-Bladder. — The facts that have been 

 enunciated above, with regard to the development of the lung in 

 Dipnoan and Crossopterygian fishes, are of much morphological 

 interest. When pieced together with what has been said regarding 

 the development of the air-bladder of Teleostean fishes they afford 

 data from which the evolutionary history of the Teleostean air- 

 bladder can be traced out with a high degree of probability. 

 That history may be 

 stated in a few words 

 to have probably 

 been as follows : 



1. The primitive 

 condition was that 

 of a lung, communi- 

 cating with the 

 pharynx by a ven- 

 trally placed glottis 

 — for we have seen 

 that the embryonic 

 rudiment of the 

 organ in the most 

 archaic forms pos- 

 sessing it is a typical 

 lung-rudiment. 



2. The organ 

 became bilobed, 

 growing back into a 

 right lung and a left 

 lung. 



3. In the forms 

 which took to a 

 purely swimming 

 existence, and became specialized in the direction of adaptation to this, 

 there came about an asymmetry of the lungs, the right lung increas- 

 ing and the left lung diminishing. Why this should have happened 

 is not yet absolutely certain : it may probably have been in adapta- 

 tion to active movements of lateral flexure, for we see the same thing 

 taking place in Gymnophiona, Snakes and Snake-like Lizards. That 

 it has been the right rather than the left lung which has increased 

 in size, is probably correlated with the rotation of this region of 

 the alimentary canal in a counter-clockwise direction as seen from 

 behind (see p. 168) which would tend to interfere more with the 

 circulation through the left lung than with that through the right, 

 by lengthening the course of the left pulmonary artery. Steps 



Fig. 98. — Sections through the lungs of a larva of 

 Polypterus 30 mm. in length. 



A, more anterior ; B, more posterior ; A, aorta ; ent, enteron ; l.l, 

 left lung ; N, notochord ; opn, opisthonephros ; p.v, pulmonary veins ; 

 r.l, right lung ; v, interrenal vein. 



