HOW ANIMALS BREATHE. 11a 
But it becomes tainted, also, by carbonic acid gas, and 
loses the free oxygen which it possessed. It is this dif- 
ference in the gaseous contents which makes the great 
difference between the crimson blocd of the arteries and 
the dark-purple blood of the veins in the higher animals. 
The First Object of Respiration is to convert venous 
into arterial blood. It is done by bringing it to the sur- 
face, so that the carbonic acid may be exhaled and oxygen 
absorbed. The apparatus for this purpose is analogous to 
the one used for circulation. In the lowest animals, the 
two are combined. But in the highest, each is essentially 
a pump, distributing a fluid (in one case air, in the other 
blood) through a series of tubes to a system of cells or 
capillaries. They are also closely related to each other: 
the more perfect the circulation, the more careful the pro- 
vision made for respiration. 
Respiration is performed either in air or in water. 
So that all animals may be classed as air-breathers or 
water -breathers. The latter are, of course, aquatic, and 
seek the air which is dissolved in the water. Land-snails, 
Myriapods, Spiders, Insects, Reptiles, Birds, and Mam- 
mals breathe air directly; the rest, with few exceptions, 
receive it through the medium of water. In the former 
case, the organ is internal; in the latter, it is more or less 
on the outside. But however varied the organs — tubes, 
gills, or lungs—they are all constructed on the same prin- 
ciple. 
(1) Sponges, Infusoria, and Polyps have no separate respir- 
atory apparatus, but absorb air, as well as food, from the 
currents of water passing through them. 
In the Star-fish, Sea-urchin, and the like, we find the 
first distinct respiratory organs, although none are exclu- 
sively devoted to respiration. There are two sets of ca- 
nals—one carrying the nutrient fluid, and the other, radi- 
ating from a ring around the mouth, distributing aérated 
