The Organs of Respiration 103 



some even climb trees for this purpose. Under the conditions 

 of the period mentioned there was a powerful inducement for 

 them to assume this habit. 



" Such conditions must have strongly tended to induce fishes 

 to breathe the air, and have acted to develop an organ for this 

 purpose. In addition to the influences of foul or muddy water 

 and of visits to land may be named that of the drying-out of 

 pools, by which fishes are sometimes left in the moist mud till 

 the recurrence of rains, or are even buried in the dried mud 

 during the rainless season. This is the case with the modern 

 Dipnoi, which use their lungs under such circumstances. In 

 certain other fresh-water fishes, of the family Ophiocephalidae, 

 air is breathed while the mud continues soft enough for the fish 

 to come to the surface, but during the dry period the animal 

 remains in a torpid state. These fishes have no lungs, but 

 breathe the air into a simple cavity in the pharynx, whose open- 

 ing is partly closed by a fold of the mucous membrane. Other 

 Labyrinthici, of similar habits, possess a more developed 

 breathing organ. This is a cavity formed by the walls of the 

 pharynx, in which are thin laminae, or plates, which undoubtedly 

 perform an oxygenating function. The most interesting member 

 of this family is Aiiabas scandens, the climbing perch. In tliis 

 fish, which not only leaves the water, but is said to climb trees, 

 the air-breathing organ is greatly developed. The labyrinthici, 

 moreover, have usually large air-bladders. As regards the occa- 

 sional breathing of air by fishes, even in species which do not 

 leave the water, it is quite common, particularly among 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 is not difficult to draw a hypothetical plan of the develop- 

 ment of the air-bladder as a breathing organ. In the two fami- 

 lies of fishes just mentioned, whose air-bladders indicate that 

 they once possessed the air-breathing function and have lost it, 

 we perceive the process of formation of an air-breathing organ 

 beginning over again under stress of similar circumstances. The 

 larval development of the air-bladder points significantly in the 

 same direction. In fact we have strong reason to believe that 



