Respiration in Invertebrate Animals. 249 



diftereut organs. No clear views as to what should and what 

 should not be characterized as distinctive of a renal organ in 

 the Invertebrate animals, have ever been defined by anatomists. 

 If a marked uniformity of structural type and plan runs 

 through the entire series of other organs in the Invertebrata, 

 such as the biliary, respiratory, circulatory, digestive, &c. sy- 

 stems, the inference is highly probable that a similar serial con- 

 sistency of plan presides over the renal system. If such be the 

 case in the Vertebrate, why should it not be so in the Inverte- 

 brate series ? In another place* the author has shown, that 

 the fluids, viewed as chemical and vital solutions, grow more 

 and more simple as the zoological scale is traced downwards 

 (or more and more complex as it is followed upwards) ; he thence 

 argues that the same tendency to simplification is also mani- 

 fested by the systems of the solid organs. This is the true 

 science of the comparative anatomy of organs. Their history in 

 this sense has never been written. If the true relation between 

 the solid machinery of the glands and the fluids could be esta- 

 blished, it would be most certainly discovered that at the point 

 in the descending series at which a given constituent of the 

 fluids, which a given gland was specially designed to withdraw, 

 ceased to exist, the gland would also cease to exist. The pro- 

 position when thus enounced assumes almost a necessary cer- 

 tainty. The mind feels at once assured that no other law can 

 explain the facts, which are indubitable. Anatomists have 

 always worked on the presumption that the fluids of the lowest 

 animal must have the same composition as that of the highest, 

 and that consequently the necessities of the organism in the 

 two instances must be the same. If the highest animal be 

 provided with a kidney, therefore the lowest must be endowed 

 with the same organ. Up to this sera in physiology, such in 

 truth has been the fallacious reasoning by which the most 

 distinguished cultivators of this science have conducted their 

 researches. The same observation applies to the secreted pro- 

 ducts of the physiological actions of organs. It is supposed 

 that because certain ingredients are found to exist under all 

 circumstances in the secretions of the higher animals, conse- 

 quently the same principles must exist in those of the lowest. 

 This false logic has led astray the minds of men for an entire 

 century. There may be nothing in common between the bile 

 of the Mammal and that of the Cephalopod, yet each may be 

 the product of the action of a liver. The same reasoning ap- 

 plies to the urinary secretion and to the renal system. Urea 

 and lithic acid, the supposed basis and essence of this secretion 



* " On the Chemistry, Physiology and Pathology of the Blood," in the 

 British and Foreign Medico-Chirurgical Review for 1853-4. 



