146 



FISHES. 



terus and the Dipnoi, in which it enters on the ventral side of 

 the oesophagus. In the majority the orifice is in the oeso- 

 phagus, but in some, as in Acipenser, in the cardiac portion 

 of the stomach, or in its blind sac, as in many Clupeoids. 

 The air-bladder may be single, or consist of two divisions 

 situated one behind the other (Fig. 52) ; its inner surface may 

 be perfectly smooth, or form manifold pouches and cells. If 





Fig. 64. — Air-bladder of Pogonias chromis. 



two divisions are present the anterior possesses a middle 

 elastic membrane which is absent in the posterior; each 

 division has a muscular layer, by which it can be separately 

 compressed, so that part of the contents of the posterior may 

 be driven into the elastic anterior division, and vice versa. 

 The posterior division being provided with the ductus pneu- 

 maticus does not require the elasticity of the anterior. 



