EVIDENCE FROM TAILS, LIMBS, <Sr* LUNGS OF ANIMALS. 117 



aperture (o). In the bony pike (Lepidosteus] it is quite as com- 

 plicated in structure, and in the Ceratodus (Fig. 50) the air-bladder, 

 whilst a single organ, exhibits a lung-like structure internally, and 

 opens into the throat by a distinct opening or glottis. But in the 

 mud-fishes or Lepidosirens (Fig. 54), which spend half the year amidst 

 dry mud and the other half in their native waters, the air-bladder 

 obtains its highest develop- 

 ment. Here it is not merely 

 double, but is also cellular 

 internally, and communicates 

 with the throat by means of a 

 tube or duct. It is, moreover, FlG S4 ._ LEPIDOSIREN OR MUD-FISH, 



provided at the extremity of 



this tube with an organ resembling the structure which guards 

 the windpipe of higher animals. The nostrils, which in other fishes 

 are simply closed pockets, open backwards in Lepidosiren into the 

 throat, and thus place the air-bladder in communication with the 

 atmosphere without. More noteworthy still, we find that part of the 

 impure blood circulating through the body of the mud-fish is sent 

 to this curious air-bladder, and circulates through its blood-vessels. 

 From the air-bladder it is returned in a pure condition to the heart, 

 and is thus fitted for re-circulation through the body. What is the 

 meaning of this curious alteration in the function and use of the air- 

 bladder ? The answer is plain. The air-bladder in the mud-fish has 

 attained its highest development. It appears as an organ receiving 

 impure blood, which is purified in its cells. It receives air from the 

 outer atmosphere for the purpose of purifying this blood. In one 

 word, the air-bladder of the fish has become a lung. 



Thus we discover that the air-bladder of the fish in reality re- 

 presents the lungs of higher animals. Evolution would proceed still 

 further, and ask us to recognise in the air-bladder the structures 

 from which the lungs have been developed in the past and a full 

 consideration of the details just presented, strengthens this latter 

 opinion. We noted that in the most primitive fishes (e.g. lancelet 

 and lampreys) no swimming-bladder was represented. Its develop- 

 ment therefore took place at a stage subsequent to the appearance 

 of the ancestors of our existing lancelets and lampreys. Gradually, 

 as the piscine type advanced, the air-bladder appeared. The fore- 

 runners of the sharks and their allies, which are as ancient as the 

 ganoids, may have possessed an air-bladder, since we find rudiments 

 of this organ in these latter fishes. But in free-swimming and 

 surface -living fishes like the sharks and dog-fishes, or groundlings 

 like the skates and rays, the necessity for a hydrostatic apparatus 

 is obviated, as it is also obviated in the flat fishes which spend their 

 lives on the sand. To the ganoids we must, therefore, look for the 



