4 34 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



ureters empty into the bladder and the urethra into the 

 cloaca. 



The diversity in structure in the different orders of 

 fishes is so great that their description would carry us 

 beyond the scope of this work. 



Invertebrates: Insects.— The essential character 

 of the kidneys— i. e., its significance in katabolism— is the 

 excretion of urea. This is therefore the true test of an 

 organ corresponding functionally to the kidneys. Now, 

 there is an extremely delicate chemical test of urea — viz., 

 the splendid purple produced by certain reagents. It is 

 wholly by this test that we are able to determine the renal 

 organ in invertebrates. In this way it is determined 

 that the so-called biliary tubes of insects (Fig. 219, bd, 

 page 336) are also their kidney — i. e., they excrete urea. 

 In crustaceans the so-called green glands (Fig. 221, 

 ggi P a g e 337)> wmc h are large glands near the base of the 

 antennae and very liberally supplied with blood, are the 

 kidneys. But it is very noteworthy that in addition to a 

 peculiar form of uric acid (carcinuric acid) there is 

 secreted also leucomaines* which is the first product of 

 albuminoid waste in higher animals, and which in them 

 is split into a combustible portion, for use as fuel, and 

 an incombustible portion excreted as urea (pages 420 

 and 430). In crustaceans some of these substances is 

 excreted unutilized, and thus wasted. This is evidence 

 of low organization. 



Mollusca. — By similar tests renal organs have been 

 detected in different classes of mollusca — viz., the 

 " spongy bodies " of cephalopods, the organ of Bojanus 

 of acephala situated at the base of the gills (Fig. 281, 

 page 402), and the lamellar gland near the pulmonic 

 sac, or the branchiae of gastropods. 



* Marchal, Rev. Sci., li, 178, 1893. 



