. Elements of Comparative Anatomy. 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 la
. Elements of Comparative Anatomy. 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 longitud
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