128 PROCEEDINGS OF THE ACADEMY OF [1885. 



degenerating organs is not uncommon. We have one instance in 

 poinl in the adaptation of the embryonal gill-arches of mammals 

 to other uses. Of these secondary einphrvments of the air- 

 bladder one seems to have some connection with the organ of 

 hearing. Another seems to be to change the direction of the 

 fish-body from the horizontal towards the vertical line. As a 

 general rule, when present, it may fix the special buoyancy of 

 the fish-bod}', and, by its situation near the back of the fish, may 

 aid to keep the dorsal surface upward in the water. This may 

 be the purpose of its lateral appendages, as the former is of its 

 longitudinal extension. Yet the fishes which have no air-bladder 

 seem none the worse off in any of these particulars. It is 

 impossible that such an organ could have developed to perform 

 functions which were satisfactorily performed without it, and it 

 seems more probable that it is an organ arrested at various 

 points in its process of degeneration, as it proved serviceable in 

 some minor function. 



If, then, we may look upon the air-bladder as an organ which 

 has partly or wholly lost its original function, the question 

 follows, what was that function ? There are certain good reasons 

 for believing that the breathing of air was the original purpose 

 of this organ. In mature Telcosteans this is occasionally indi- 

 cated by the existence of a pneumatic duct connecting with the 

 (esophagus. It is true that this duct is usually of no functional 

 use. and varies from partial to complete disappearance. But the 

 fact is, that all fishes with an air-bladder possess a duct in the 

 early stage of embryological development. In the mature stage 

 it is lost by all Teleosteans except the Physostomes. 



Thus embryological evidence indicates that one original function 

 of the air-bladder was the introduction of external air into the 

 body, a function which has now lost its importance. And the 

 apparatus for compressing and dilating the bladder may have 

 been originally developed as an aid in this function. Also the 

 extraordinary development of retia mirabilia, in the inner tunic 

 of many air-bladders, now used only to secrete gas into the 

 interior, may be a survival of ancient pulmonary capillaries, 

 which have changed their character with their function. 



There are other reasons beyond those here given thai the air- 

 bladder was originally an air-breathing organ. Embryology 

 points back to the condition of the primal fishes. But of these 



