RESPIRATION 49 



and situated close to the backbone (Fig. 2ob). The Cuchia, 

 which grows to a length of about two feet, bears a superficial 

 resemblance to some of the Eels, to which, however, it is not 

 at all related. Like the other fishes just mentioned, it is an 

 inhabitant of the fresh and brackish waters of India and Burma, 

 and spends much of its time in the grass on the banks of ponds 

 like a snake. The air-breathing organs consist of a pair of sacs 

 growing out from the pharynx above the gills. This curious fish 

 seems to have lost practically all its power of aquatic respiration, 

 for when in the water it is forced to come to the surface at 

 frequent intervals to gulp air, and the true gills are very much 

 reduced, being represented by a few rudimentary filaments 

 attached to the second of the three remaining gill-arches. 



The accessory respiratory organs just described are not the 

 only structures used for air-breathing among fishes, and before 

 concluding this chapter some more lung-like organs must be 

 considered. The air-bladder, or, as it is sometimes called, the 

 swim-bladder, must be a familiar object to those who have had 

 occasion to examine the inside of a fish. Situated within the 

 body cavity, and immediately below the backbone, it generally 

 has the appearance of a long, cylindrical bag with glistening 

 silvery walls. It is very variable both in size and form in 

 different fishes, and may be present in one species and entirely 

 absent in a closely related form. That it normally contains 

 air or gas of some kind may be readily demonstrated by punc- 

 turing it with a needle, when the walls promptly collapse. 

 There is no other single organ in any group of vertebrates 

 which performs such a variety of functions as does the air- 

 bladder of Bony Fishes. In the majority it serves as a hydro- 

 static organ or float, enabling its possessor to accommodate 

 itself to the varying pressure encountered at different depths 

 [cf. p. 174), in others it is an organ for the production of sound 

 {cf. p. 156), and in others, again, it is more or less connected 

 with the sense of hearing {cf. p. 193). For the present it must 

 suffice to consider its relation to air-breathing and its con- 

 nection with the lungs of higher vertebrates. 



Like the lungs of a man, the air-bladder is intimately asso- 

 ciated with the alimentary canal, and a study of the develop- 

 ment of this organ shows that it begins as a minute pouch 

 budded off from the gullet. This gets larger and larger, until 

 it is finally separated off from the gullet, remaining connected 

 only by a narrow tube known as the pneumatic duct. In some 

 fishes this duct remains open throughout life, but in others it 



