132 PROCEEDINGS OF THE ACADEMY OF [1885. 



mucous membrane. In the family Labyrinthici the accessory 

 breathing cavity becomes an organ, with thin lamina 1 or plates, 

 which undoubtedly perform an oxygenating function. This organ 

 is greatly developed in Anabas scandens, the Climbing Perch. In 



addition t<> these there are eases in which fish have drying pools, 

 and migrate for a considerable distance overland in search of 

 water, with no breathing organ but the gills. 



[f even now, when the land is everywhere occupied with active 

 and dangerous foes, so many fish find occasion to venture on 

 shore, it i- quite probable that in the early period, when it conld 

 he visited without danger, very many lishes may have paid tem- 

 porary visits to the land. And if now. under this influence, and 

 that of drying pools and stagnant water, many fish have acquired 

 a partial air-breathing habit, this was far more likely to take 

 place under the more favorable conditions of ancient times. It 

 seems quite possible that the development of the air-bladder was 

 due to influences of this character. The occasional habit of 

 breathing air is quite common with fish, especially of fresh-water 

 species. Cuvier remarks that air is perhaps necessary to every 

 kind of fish ; and that, particularly when the atmosphere is warm, 

 most of our lacustrine species sport on the surface for no other 

 purpose. 



It may be even possible to draw a hypothetical scheme of the 

 original process of development of the air-bladder as a breathing 

 organ. Embryology indicates that its existence began in an 

 eversion of the intestinal canal, in its (esophageal portion, and 

 that this gradually became an air-bladder with its pneumatic duct. 

 It may have had its primal form in a simple pharyngeal cavity, 

 like that of the Ophiocephaluhe. partly closed otf from the food- 

 passage by a fold of the mucous membrane. A step further 

 would reduce this membraneous fold to a narrow opening, leading 

 to an inner pouch. From such a condition the development of 

 the Ganoid air-bladder, with its pneumatic duct of greater or less 

 length, is a probable and natural one, and is sustained by embryo- 

 logical evidence. Though we do not possess the intermediate 

 steps, and the breathing organ of the Labyrinthici is a specialized 

 apparatus aside from this line of progress, yet the breathing pouch 

 of the Ophiocephalida' is in the direct line of development of the 

 Ganoid air-bladder. We can scarcely look upon it as in any 

 sense a survival of the archseic air-breathing organ. It is more 



