RESPIRATORY ORGANS. 



249 



common, the form being most varied in the physoclistous species. In- 

 ternally the walls may be smooth and the cavity simple, or it may be sub- 

 divided by septa (fig. 257), or, as in Amia and Lepidosteus, it may be 

 alveolar, recalling the condition in the lungs of higher vertebrates. 

 The walls sometimes contain striated muscle, and in some siluroids and 

 cyprinoids they are more or less calcified, partly by the inclusion of 

 processes from the vertebrae. 



FIG. 257. Ventral view of opened air bladder and Weberian apparatus of Macrones, 

 combined from Bridge and Haddon. a, atrial cavity; ac, anterior chamber of air bladder, 

 the arrows showing the connexion with the posterior chamber; de, endolymph duct; s, 

 sacculus; sc, scaphium; sk, sub vertebral keel; tra, trc, anterior and crescentic processes of 

 tripos; M, utriculus. 



The blood supply is arterial, coming from either the aorta or the 

 coeliac axis, in some instances different portions receiving blood from 

 both. In the walls the arteries break up into networks of minute 

 vessels ('rete mirabile'), these frequently making 'red spots 1 on the 

 inner surface. From the retia the blood passes to the body veins, (post- 

 cardinal, hepatic or vertebral). In the ganoids and phystomous species, 

 especially those with a wide pneumatic duct, the gases contained in the 

 swim bladder may be obtained directly from the air or water, but in the 

 physoclists this is impossible and the red spots may be the place of its 



