262 



BONY FISHES 



IX. 11 



Saccobranchns there are large air sacs growing a long way down the 

 body from the gill chambers (Fig. 156). 



The air-bladder, which has contributed so largely to the success of 

 the later teleosts, may have arisen as an accessory respiratory organ, 



STURGEON AND 

 MANY TELEOSTS 



LEPIDOSTEUS 

 g= AND AMIA 



ERYTHRINUS 



CERATODUS 



POLYPTERUS AND 

 CALAMOICHTHYS 



LEPIDOSIREN AND 

 PROTOPTERUS 



REPTILES 



BIRDS 



MAMMALS 



Fig. 157. Air-bladder of various fishes, seen from in front and from the left side. 



A, air- or swim-bladder; ad, air-duct; D, digestive tract. (From Dean, Fishes, Living & 



Fossil, The Macmillan Co., after Wilder.) 



used in the same way as those described above. In all the more 

 primitive teleosts (Isospondyli) the air-bladder preserves in the adult 

 its opening to the pharynx ('physostomatous'), whereas in higher 

 forms it becomes completely separated ('physoclistous'). Survivals of 

 still earlier Actinopterygii have the opening especially well developed, 

 though it varies from group to group (Fig. 157). Thus in the stur- 

 geons there is a wide opening into the dorsal side of the pharynx. 

 In Amia and Lepisostens the opening is also dorsal and the walls of 

 the sac are much folded and used for respiration. In Polypterus 

 the opening is ventral and the bladder has the form of a pair of lobes 

 below the gut. This arrangement recalls that of the tetrapod lungs and 



