568 



COMPAEATIVE ANATOIMY. 



single genera. Thus, some species of tlie genus Scomber do, and 

 others do not, possess an air-bladder. 



There are gi'eat variations in the mode of connection between 

 the air-duct and the gut. It may open at the sides, or above, 

 and in all regions of the fore-gut from the pharynx as far as the 

 end of the stomach. It varies very greatly in form also. In the 

 Cyprinoids it is divided transversely into two poi'tions which lie 

 one behind the other, the air-duct being given off from the hinder 

 one (of. Fig. 301, in n). In others there are lateral diverticula, 

 which may become simple or branched processes (Fig. 321, B C a). 



The air-duct, which is 



A 



li 



J 



often very narrow and 

 long in the Physostomi, 

 is ill adapted for the 

 passage of air; in the 

 Physoklisti air cannot of 

 course be taken in in 

 this way. In the latter, 

 therefore, the air in the 

 air-bladder must be re- 

 garded as secreted from 

 the walls of the bladder, 

 while in many Physos- 

 tomi the air-duct can 

 only serve as an occa- 

 sional outlet for this air. 

 In texture the walls of 

 the organs resemble those 

 of the gut, but there 

 are at the same time a 

 number of special dif- 

 ferentiations which it is 

 beyond our purpose to 

 speak of here. The various adaptations of the air-bladder to other 

 organs, as, for example, its connection with the auditory organs of 

 many Physostomi, are differentiations of this kind (cf. supra, § 100). 

 In the DipnoT the air-bladder is more lung-like in character. 

 Although in its external characters the organ is just like an air- 

 bladder, yet there is an essential difference owiug to the presence on 

 it of afferent veins and efferent arteries ; and, owing to this change, 

 the air-bladder is henceforward to be regarded as a respiratory 

 organ. In Ceratodus, where, indeed, it only occasionally functions 

 as a lung, it is formed of a single sac, which extends along the 

 whole of the dorsal region of the coelom, and presents indications 

 merely of longitudinal division; in Lepidosiren and Protopterus 

 it is divided into two halves. 



Fig. 321. Various forms of air-bladders. A Of 

 Polypterus bichir (after J. Muller). B Of 

 Johnius lobatus. C Of Corvina trispinosa 

 (after Cuvier and Valenciennes) . a Appendages of 

 the air-bladder. 6 Its orifice. 



