XXIII.] THE CAUSES OF DEVELOPMENT. 299 



I uow come to the organs of respiration ; and I have to Eespha- 

 begin this subject with the remark, that among the lower organs: 

 tribes of animals geuerallv, those oroans are surprisingly t^'^!'', .,. 



^ ^ ^ *^ , . vaiiaLility. 



inconstant, not only as to their position, but as to their 

 existence. This, I think, is more remarkably the case 

 among the water-breathing Gasteropodous Mollusca than 

 in any other class, but it is generally true of the aquatic 

 Invertebrata ; and in close connexion with this is the fact, 

 that the most remarkable cases in the animal kingdom 

 of organs that assume new functions totally unlike their 

 original ones are among respiratory organs. This extra- 

 ordinary plasticity of the respiratory organs is evidently 

 connected with, and I believe is a result of, the fact that 

 among the lowest animals, and in some degree among the 

 highest, the function of respiration belongs to the whole 

 surface. " The alimentary canal respires, digests, and 

 excretes in the larva of the dragon-fly and in the fish 

 Cobites," ^ neither of which is a very low organism. Among 

 the Batrachia a great proportion of the respiration takes 

 place through the sldn ; ^ in the axolotl, which is one of 

 that class, the importance of the branchi^ is so slight that 

 their removal does not appear to injure the animal, or even 

 to increase the necessity for coming to the surface of the 

 water in order to breathe air ; ^ and even in man there is a 

 slight amount of respiration through the skin.* I believe 

 it may be safely asserted that all land animals have distinct 

 respiratory organs.^ 



Eespiration, though it takes place in living beings, is Eespira- 

 essentially a merely chemical and physical process ; it physical 

 consists in absorbing oxygen, and giving out the carbonic process. 



1 Danvin's Origin of Siiccies, p. 220. 



2 Carpentor's Human Physiology, p. 293. 



3 Memoir by Augnste Dumeril, translated in the Annals of Natural 

 History, December 1867. 



* Carpenter's Human Physiology, p. 293. 



* Those of the earthworm have remained unknown till lately. But it is 

 now stated that its " respiratory aj)paratus is an aqueous sac, lined with 

 vibratile cilia, within the abdominal cavity, on either side of the body." 

 (Dr. Coote on Nerve Structure and Force, Quarterly Journal of Science, 

 April 1867.) The respiratory organs of the laud mollusca are well known. 



