120 COMPARATIVE ZOOLOGY. 
CHAPTER XV. 
SECRETION AND EXCRETION. 
In the circulation of the blood, not only are the nutrient 
materials deposited within the body in the form of tissue, 
but certain special fluids are separated and conveyed to 
the external or internal surfaces of the body. ‘These flu- 
ids are of two kinds: some, like saliva, gastric juice, bile, 
milk, ete., are for useful purposes; others, like sweat and 
urine, are expelled from the system as useless or injurious. 
The separation of the for- 
mer is called secretion; the 
removal of the latter is ea- 
cretion. The two functions 
differ also in this, that se- 
cretion is intermittent, as, e. 
g-, the gastric juice is form- 
ed when wanted, while ex- 
cretion is constant day and 
night. Both processes, how- 
ever, are substantially alike. 
In the lowest forms, there 
are no special organs, but 
secretion and excretion take 
Bre, 80.«-Three plans of secreting Mem wsiace fromthe @eneral us 
branes. The heavy line represents the 
areolar-vascular layer; the next line is face. Even in the higher 
the basement, or limiting membrane; : a 
and the dotted line the epithelial layer: animals, there are secreting 
a@ shows increase of surface by simple 1 TI ; 
plaited or fringed projections; 0, five membranes. 1e@ mem- 
modes of increase by recesses, forming SAT OF } 
simple glands, or follicles; c, two forms branes lining the nose and era 
Of compe ya ands. alimentary canal and inclos- im 
ing the lungs, heart, and joints, secrete lubricating fluids. ~ 
