126 COMPARATIVE ZOOLOGY. 



more immediately necessary to life than any other : the 



arrest of respiration is fatal. 



2. While the lungs (and skin also, 

 to a slight degree) are sources of 

 gain as well as loss to the blood, the 

 kidneys are purely excretory organs. 

 Their main function is to eliminate 

 the solid products of decay which 

 cannot pass out by the lungs. In 

 Mammals, they are discharged in 

 solution ; but from other animals 

 which drink little the excretion is 

 more or less solid. In Insects, the 

 kidneys are groups of tubes ; in the 



F,. 93. -section of Human hi g ber Mollusks, they are represent- 

 Kiduey, showing the tubu- e d by spongy masses of follicles : in 



lar portion, 3, grouped into '* Dl/ ' 



cones; 7, the ureter, or out- vertebrates, they are well-developed 



let of the secretion. i j u j 



glands, two in number, and consist- 

 ing of closely packed tubes. 



3. The skin of the soft-skinned animals, particularly of 

 Amphibians 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, a'nd 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 Mam- 

 mals; the milk of Mammals; the ink of the Cuttle-fish; 

 the poison of Jelly-fishes, Insects, and Snakes; and the 

 silk of Spiders and Caterpillars. 



