PHYSIOLOGY OF THE INVERTEBRATA. 287 



These results are expressed in milligrammes of urea per 

 100 grammes of muscular tissue.* 



It is most probable that the formation of urea takes place 

 in the muscles. It is certainly present in the blood of Mya 

 and Anodonta. Milne-Edwards states that "it is probable 

 that in all cases the secreted matter exists in the blood 

 already formed. It was thought, for example, that the urea 

 found in urine must be formed by and in the kidneys, since 

 it could not be detected by chemical analysis in the blood ; 

 but if these organs be destroyed in a living animal, or re- 

 moved, urea will, after a certain time, be formed in the blood, 

 thus clearly proving that the kidneys do not form it." 



In the higher animals an abundant alimentation gives rise 

 to a greater excretion of uric acid and urates. On the con- 

 trary, in abstinence the uric acid and its salts disappear, but 

 urea is excreted in greater quantity. This applies not only 

 to Vertebrates but also to many Invertebrates. Urea is a 

 product of more or less complete oxidation of organic sub- 

 stances, and is formed, as already stated, in muscular tissues, 

 by the disintegration of the anatomical elements. Uric acid, 

 on the other hand, is the result of an incomplete oxidation, 

 and is produced for the most part in the bood or its equivalent, 

 when such fluid is surcharged with peptones which the tissues 

 are unable to assimilate. Secretion and excretion can be 

 traced back to the phenomena of nutrition — that is to say, to 

 the molecular acts effected in the midst of glandular cells, 

 which means that it can be accomplished without the inter- 

 vention of the nervous system. Such is the case with the 

 lowest Invertebrates ; but in the higher forms, possessing a 

 more or less complete nervous system, secretion and excretion 

 are largely influenced by nerves. It may be probable that 

 if the commissural cords connecting the supra-cesophageal 

 with the sub-CBSophageal ganglion were severed nervous 

 stimulus would not be supplied to the green glands of 



* See also Smith's new method for estimating urea in thePAorm. Journ. 

 Trans. [3], vol. 21, p. 294. 



