92 ESSAY ON CLASSIFICATION. 



again how unsound it is. The organs of respiration and 

 circulation of fishes afford a striking example. How long 

 have not their gills been considered as the equivalent of 

 the lungs of the higher Vertebrata, merely because they 

 are breathing organs ; and yet these gills are formed in a 

 very different way from the luogs : they bear very dif- 

 ferent relations to the vascular system ; and it is now 

 known that they may exist simultaneously with lungs, as 

 in some full-grown Batrachians, and, in the earlier em- 

 bryonic stages of development, in ah 1 Vertebrata. There 

 can now no longer be any doubt that they are essentially 

 different organs, and that their functions afford no test of 

 their nature, and cannot constitute an argument in favour 

 of their organic identity. The same may be said of the 

 vascular system of the fishes. Cuvier 1 described their 

 heart as representing the right auricle and the right ven- 

 tricle, because it propels the blood it contains to the gills, 

 in the same manner as the right ventricle propels the 

 blood to the lungs of the warm blooded animals ; yet 

 embryology has taught us that such a comparison, based 

 upon the special relations of the heart of fishes, is un- 

 justifiable. The air-sacs of certain spiders have also been 

 considered as lungs, because they perform similar respi- 

 ratory functions ; and yet they are only modified tracheae, 2 

 which are constructed upon such a peculiar plan, and stand 

 in such different relations to the peculiar kind of blood of 

 the Articulata, 3 that no homology can be traced between 

 them and the lungs of Vertebrata, any more than between 



1 CUVIER (G.), Regn. Anim., 2de 3 BLANCHARD (EM.),De la circula- 

 6'lit., vol. ii, p. 122. tioii dans les lusectes. Compt. Rend., 



2 LEUCKART (R.), Ueber den Bau 1847, vol. 24, p. 870. AGASSIZ (L.), 

 und die Bedeutung der sogenannten On the Circulation of the Fluids in 

 Lungen bei den Arachniden, in SIE- Insects, Proceedings of the Arne- 

 BOLD und KOLLIKER'S Zeitschrift, f. rican Association for 1849, page 

 wiss. Zool., 1849, I, p. 246. 140. 



