452 



ANIMAL RESPIRATION 



lengthy excursions on land, while it is even accredited with the 

 power of climbing- trees, and has been named in accordance with 

 this belief. 



The arrangements described for the last two sorts of fish, 

 though extremely interesting, do not form a stage in the evolu- 

 tion of the backboned animals which live on land. And we 

 now proceed to the consideration of those fishes which do throw 



Fig. 564. — Climbing Perch {Anabas 

 scaiidens). Side of head dissected to 

 show accessory breathing organs. 



Fig. 565. — Dissection of a Bitterling {RJiodeiis aniains) 



bl, Swim-bladder narrowed in the middle; j, gullet; </, intestine: 

 // liver; "'and /z, auricle and ventricle of heart; ?i, kidney; e, ovary; 

 Ig, egg-laying tube (rarely present in fishes). 



some light on the way in which such forms have originated 

 from aquatic ancestors. 



Evolution of the Lungs of Backboned Animals. — It has 

 already been pointed out (see p. 421) that the lungs of a Frog, 

 Lizard, or other air-breathing land-vertebrate are comparable to 

 the swim-bladder possessed by many fishes, serving in them to buoy 

 up the body and helping to maintain its equilibrium (fig. 565). 

 The swim-bladder is developed as an outgrowth from the front 

 part of the digestive tube, with which it often remains connected 

 throughout life by means of a pneumatic duct. It contains a 

 mixture of nitrogen and oxygen, the latter gas being more 

 abundantly present in freshwater species. In some of the Ordi- 

 nary Bony Fishes (Teleosts) it has been shown that air is actually 

 taken into this organ from the exterior, and that this furnishes 

 what may be called a new method of breathing, supplemental to 

 that of the gills. Indeed it has been proved in some such cases 

 that the gills alone cannot do all the necessary work of respiration. 

 For instance, it has been found that in some of the freshwater 

 fishes of tropical South America (species of Erythrinits) suffocation 

 quickly takes place if air be prevented from entering the swim- 

 bladder. 



