THE BREATHING APPARATUS. 333 



teresting and remarkable discovery is now made that even 

 the permanent respiratory organ of the higher Vertebrates, 

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

 section 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, which 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, 1C, 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 



