107 



those seen in the allantois of the frog. Now, the allantois in this 

 animal, like the same membrane in the incubated egg, is now allowed 

 to perform a respiratory office, and this was first described by Town- 

 son, in his work entitled ' De Amphibiis,' which was published more 

 than half a century ago ; so that in one and the same animal we have 

 two structures exhibiting an analogy in the arrangement of their ves- 

 sels to two others, whose true functions are known. On exhibiting 

 one of these preparations to my friend Mr. Dalrymple, he at once 

 pronounced that the vessels approached more nearly to the distribu- 

 tion in the cellular lungs of reptiles, than to any other structure that 

 he was acquainted with ; and it is more than probable, if the air- 

 bladder of the Lepidosteus before alluded to, were injected, that the 

 analogy would be so striking, that it would be difficult to determine 

 which was air-bladder and which lung. There are many other fish 

 which have air-bladders equally interesting, in their stomachs, and 

 which approach very nearly the cellular lungs of the Reptilia. I may 

 notice, besides the Lepidosiren and Lepidosteus, that of the Silurus 

 felis, which is sacculated like the lung of a frog. A figure has been 

 given of this air-bladder by Cuvier, in his ' Comparative Anatomy.' 



From the present state of our knowledge, it is difficult to say why 

 the air-bladder should be absent in so many species of the finny race, 

 and present in others. It would require a most rigid examination into 

 the intimate structure and habits of those fish that are not supplied 

 with this organ, to answer this question : it will, no doubt, be found 

 eventually, that there is some arrangement either in the gills, or some 

 other part of their system, to compensate for this loss, and, at the 

 same time, prove the unity of the plan on which all these remarkable 

 animals are constructed. 



Since writing the above, I have succeeded in injecting the lungs of 

 the chameleon, and in this animal the arrangement comes nearest that 

 found in the upper compartment of the air-bladder of the eel, the 

 only differences being, that in the chameleon the capillaries are much 

 smaller, and the formation of the meshes more angular, than in the 

 eel, as seen in figs. 1 and 6 (PL xii.), as compared with fig. 2. In 

 other respects they are perfectly identical. I may here also state, 

 that at Professor Owen's request, I succeeded in injecting a small 

 portion of the air-bladder of the Lepidosiren, and the arrangement 

 of the vessels was precisely like that of the lungs of the Reptilia. 

 July 19, 1842. 



