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 (Figs. 
Fig. 93 .—section or H»mau 4 M2); in the higher Mollnsks, they 
Kidney, showing the tuba- are represented by spongy masses of 
lar portion, 3, grouped into ... . . . \ T 
cones; 7, the ureter, or out- follicles (I ig. 46) ; in Vertebrates, 
let for the secietion. they are well-developed glands, two 
in number, and consist 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, 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 Keptiles, 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. 
