lo THE POPULAR SCIENCE MONTHLY. 



so as to resemble a frog's iungs ; and the walls and partitions of these 

 cells have many blood-vessels. These air-bladders are, in fact, more 

 cellular and more vascular than the lungs of Menohranchus^ or the 

 hinder and larger portion of the lungs of serpents. And, in the light 

 of the observations already recorded, there seems good reason for be- 

 lieving that pure air is inhaled and vitiated air exhaled whenever the 

 fish rises to the surface. 



It is worth noting, also, that both Amia and Lepidosteus are very 

 tenacious of life, and endure removal from the water for a time much 

 better than do the sturgeons, whose air-bladders are neither cellular 

 nor vascular. The latter, also, are bottom-feeders, while the gars seem 

 to keep near the surface of the water. 



Why, then, are not these air-bladders lungs ? 



The most obvious objection is, that their openings are into the 

 upper or dorsal side of the throat, while the glottis of batrachians, 

 reptiles, birds, quadrupeds, and ourselves, communicates with the 

 lower or ventral side. 



This objection may be met in two ways. In the first place, if al- 

 lowed, we should have to admit that all the so-called air-breathing 

 vertebrates have organs (the lungs) which have no representative in 

 the fishes, and that most of the latter have an organ (the air-blad- 

 der) which has no representative in the former. 



It is true that some fishes have no air-bladder ; but with some, as 

 Amphioxtis, the lamprey-eels, the sharks,* and the skates, we may in- 

 fer that it has not yet become developed ; while with others, as the 

 fiat fishes, the air-bladder may have been lost through what may be 

 called a local retrograde metamorphosis. 



It is important to note, also, that an air-bladder and lungs have 

 never been found in one and the same animal ; and since arms, front- 

 legs, flippers, and wings, are all regarded as modifications of the same 

 organs, anterior limbs ; and since, in many other cases, organs of 

 very different size, form, complexity, and function, are considered as 

 homologous, we shall be following precedent in admitting a willing- 

 ness to regard air-bladders and lungs as modifications of the same 

 organ. 



But the true argument against the objection is derived from the 

 existence of transition forms, or links, between air-bladders and lungs, 

 as to the position of the organs themselves, and their communications 

 with the alimentary canal. 



"With Amia and Ijepidostetis the air-bladder and the opening of 

 the duct are both dorsal. With the Brazilian fish called Jri/thrim(S 

 (as first stated by Johannes Milller, and lately verified by the writer), 

 the duct opens iipon one side of the throat. In the lately-discov- 

 ered Ceratodus of Australia, as described by Gtlnther, the sac and duct 

 are single, but the former is vascular, and the latter enters at the left 



' Maclay has figured a rudimentary air-bladder in certain ehark-embryos. 



