334 



COMPARATIVE ZOOLOGY 



neys are purely excretory organs. Their main function 

 is to eliminate the solid products of decay which can not 

 pass out by the lungs. In mam- 

 mals, they are discharged in solu- 

 tion ; but from other animals 

 which drink little the excretion 

 is more or less solid. In insects, 

 the kidneys are groups of tubes 

 (Figs. 239, 240); in the higher 

 mollusks, they are represented 

 by spongy masses of follicles 

 (Fig. 244); in vertebrates, they 

 are well-developed glands, two 

 in -number, and consist of closely 

 packed tubes. 



3. The skin of the soft-skinned 

 animals, particularly of amphibi- 

 ans and mammals, is covered with 

 minute pores, which are the ends of as many delicate 

 tubes that lie coiled up into a knot within the true skin. 

 These are the sweat glands, which excrete water, and 

 with it certain salts and gases. 



Besides these secretions and excretions, there are 

 others, confined to particular animals, and designed for 

 special purposes : such are the oily matters secreted 

 from the skin of quadrupeds for lubricating the hair and 

 keeping the skin flexible; the tears of reptiles, birds, 

 and mammals; the milk of mammals'; the ink of the 

 cuttlefish ; the poison of jelly fishes, insects, and snakes ; 

 and the silk of spiders and caterpillars. 



FIG. 290. Section of Human 

 Kidney, showing the tubular 

 portion, 3, grouped into cones; 

 7, the ureter, or outlet for the 

 secretion. 



