Chap, viii.] SPECIAL SECRETIONS. 265 



The two important constituents of the urine of the 

 Vertebrata are urea and uric acid ; the former is 

 the preponderating constituent in the Mammalia, 

 the latter in the Sauropsida, and, as urea is readily 

 soluble in water, while uric acid is very insoluble, we 

 find that the renal products of the Sauropsida are or- 

 dinarily semi-fluid, and dry rapidly on exposure to 

 the air. The urine of carnivorous mammals is more 

 concentrated and more acid than that of man ; that 

 of herbivorous forms is ordinarily alkaline, but when 



/ 



it is acid in reaction, uric acid is as abundant as 

 in the lion or the tiger (Garrod) ; the herbivora have 

 a large quantity of hippuric acid, which is only found 

 in small quantities in man. 



CHAPTER VIII. 



ORGANS OF SPECIAL SECRETIONS. 



addition to the various secretions, such as saliva 

 and bile, or excretions such as the uric, there are 

 others which, though dependent, of course, 011 the 

 activity of protoplasmic cells, are special and peculiar 

 to different animals, and are not a necessary result of 

 protoplasmic activity ; such, for example, is the poison 

 of the scorpion, or the ink of the cuttlefish. 



Poison or venom glands* While in the 

 Ophidia or the mad dog the poison is due to a modifi- 

 cation of the proper salivary glands, we find special 

 glands developed in various Arthropods. Among the 

 Arachnida, the spider is provided with tubular 

 glands placed at the base of the chelicerse, or first pair 

 of appendages, which open by a narrow duct at the 

 orifice at the end of these organs ; the two last joints 

 are movable on one another, and are thus enabled to 



