io6 The Organs of Respiration 



' ' Under the hypothesis here presented the process of evolu- 

 tion involved may be thus summed up. Air-breathing in fishes 

 was originally performed by the unchanged walls of the oesoph- 

 agus perhaps at specially vascular localities. Then the wall 

 folded inward, and a pouch was finally formed, opening to the 

 air. The pouch next became constricted off, with a duct of con- 

 nection. Then the pouch became an air-bladder with respira- 

 tory function, and finally developed into a simple lung. These 

 air-breathing fishes haunted the shores, their fins becoming con- 

 verted into limbs suitable for land locomotion, and in time 

 developed into the lung- and gill-breathing batrachia, and 

 these in their turn into the lung-breathing reptilia, the loco- 

 motor organs gradually increasing in efficiency. Of these pre- 

 batrachia we have existing representatives in the mud-haunt- 

 ing Dipnoi, with their feeble limbs. In the great majority of 

 the Ganoid fishes the bladder served but a minor purpose as a 

 breathing organ, the gills doing the bulk of the work. In the 

 Teleostean descendants of the Ganoids the respiratory function of 

 the bladder in great measure or wholly ceased, in the majority 

 of cases the duct closing up or disappearing, leaving the pouch 

 as a closed internal sac, far removed from its place of origin. 

 In this condition it served as an aid in swimming, perhaps as a 

 survival of one of its ancient uses. It gained also in certain 

 cases some connection with the organ of hearing. But these 

 were makeshift and unimportant functions, as we may gather 

 from the fact that many fishes found no need for them, the 

 bladder, in these cases, decreasing in size until too small to be of 

 use in swimming, and in other cases completely disappearing 

 after having travelled far from its point of origin. In some other 

 cases, above cited, the process seems to have begun again, in 

 modern times, in an eversion of the wall of the oesophagus for 

 respiratory purposes. The whole process, if I have correctly 

 conceived it, certainly forms a remarkable organic cycle of de- 

 velopment and degeneration, which perhaps has no counterpart 

 of similarly striking character in the whole range of organic 

 life." 



The Heart of the Fish. The heart of the fish is simple in 

 structure, small in size, and usually placed far forward, just 

 behind the branchial cavity, and separated from the abdominal 



