268 BIOLOGY. [BOOK n. 



Organs analogous by function to the kidneys of the superior 

 vertebrates exist in many invertebrates, notably in insects and 

 the arachnida. In these animals the secretory cells are even 

 sometimes very voluminous. Those of the Coccus ffesperidum, 

 for instance, are so large that they can only place themselves in 

 single file in the vessel of Malpighi, to which they give a knotted 

 aspect. These uriniferous cells dissolve and set at liberty the 

 numerous granulations of uric acid and of urate which they 

 contain. 



The schema of the kidney of the vertebrates is found in the 

 myxinoids. 1 The apparatus is composed of a long canal com- 

 parable with the ureter, that is to say, with the long and 

 narrow conduit, which, in the superior vertebrates, unites the 

 kidney to the bladder. In the myxinoids this canal shows from 

 distance to distance culs-de-sacs narrowed at the neck. At 

 the bottom of every cul-de-sac is found one of those small 

 clusters, consisting of intricated capillaries, which are called 

 glomerules of Malpighi, in the kidney of the superior vertebrates 

 (Fig. 38). This arrangement of the renal capillaries has for 

 result to slacken the course of the blood in the glands, to 

 multiply the contacts of the vessels with the excretory cells, 

 and consequently it is very suitable for facilitating the urinary 

 excretion. Tn effect this excretion is of supreme importance, 

 forasmuch as it represents a grand current of expulsion, thanks 

 to which the principal mass of the mineral principles and of the 

 quaternary azotised principles, disassimilated, useless or hurtful, 

 is driven from the animal economy. 



The urinary liquid represents the residuum the most general 

 and most abundant of disassimilation, that is to say, that its com- 

 position is alike very complex and very variable. With the 

 exception of a small quantity of colouring matter or some pro- 

 ducts of vesical secretion in the animals that have a bladder, we 

 find in it no immediate principles of the third class. It is 

 merely a solution much charged with mineral or mineralised 

 1 Leydig, loc. cit. 



