59- 



STRUCTURE OF VERTEBRATES 



Structure, from which the swim-bladder was derived. The origin 

 of the Actinopterygii was probably in fresh water, but their 

 great expansion was in the sea ; here there was no shortage of 

 oxygen, and the lung, developed for respiration in marshes or 

 sluggish rivers, could be put to hydrostatic uses. It is doubtful 



STURGEON AND 

 MANY TELEOSTS 



LEPIDOSTEUS 

 AND AMIA 



ERYTHRINUS 



:<^\j\nTTinnnnn 



mi 



CERATODUS 



POLYPTERUS AND 

 CALAMOICHTHYS 



LEPIDOSIREN AND 

 % PROTOPTERUS 



REPTILES 



BIRDS 



MAMMALS 



Fig. 460.— The air-bladder of various fishes, seen from in front and from the 

 left side.— From Young, The Life of Vertebrates, 1950. Clarendon Press, 

 Oxford. After Dean and Wilder. 



if any teleost uses its air-bladder as a lung, although some of 

 the primitive physostomatous forms may do so. Many teleosts 

 have taken to breathing air, but they do so either, Uke the eel, 

 through the general body surface, or through specially developed 

 structures such as the air chamber above the head in the climbing 

 perch {Anabas) of India. In many fiat fishes, which neither 

 breathe air nor change their level in the sea, there is no air-bladder 

 at all. 



