THE r.REATIIING APPARATUS. 333 



fceresting and remarkable discovery is now made that oven 

 the permanent respiratory organ of the higher Vertebrates, 

 the air-breathing lungs, lias also developed from this anterior 

 Bection of the intestinal canal. Our lungs, together with 

 the wind-pipe {trachea) and the larynx, develop from the 

 ventral wall of the anterior intestine. This entire great 

 breathing-apparatus, wliich occupies the greater part of 

 the chest (thorax) in the developed Man, is at first merely 

 a very small and simple vesicle or sac, which grows out 

 from the intestinal canal immediately behind the gills, and 

 soon separates into two lateral halves (Figs., 284, c, 285, c ; 

 Plate V. Figs. 13, 15, 16, lu). This vesicle occurs in all 

 Vertebrates except in the two lowest classes, the Acrania and 

 Cyclostomi. In the lower Vertebrates, however, it develops, 

 not into lungs, but into an air-filled bladder of considerable 

 size, occupying a great part of the body-cavity (coeloma), 

 and which is of quite a different significance from the 

 lungs. It serves, not for breathing, but as an hydrostatic 

 apparatus: for vertical swimming movements it is the 

 swimming-bladder of Fish ; but the lungs of Man and of 

 all other air-breathing Vertebrates develop from the same 

 simple bladder-like appendage of the anterior intestine, 

 which, in Fishes, becomes the swimming-bladder. 



Originally this sac also has no respiratory function, but 

 serves only as an hydrostatic apparatus, augmenting or 

 diminishing the specific gravity of the body. Fishes, in 

 which the swimming-bladder is fully developed, are able to 

 compress it, and thus to condense the air contained in it. 

 The air sometimes also escapes from the intestinal canal 

 through an air-passage which connects the swimming- 

 bladder with the throat {^pharynx), and is expelled through 



