112 
COMPARATIVE ZOOLOGY. 
gases between the animal and its surroundings is called 
Respiration. 
The First Object of Respiration is to convert venous 
into arterial blood. It is done by bringing it to the sur¬ 
face, so that carbon dioxide may be exhaled and oxygen 
absorbed. The apparatus for this purpose is analogous to 
the one used for circulation. In the lo’west 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 Mammals 
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 out¬ 
side. But however varied the organs—tubes, gills, or lungs 
—they are all constructed on the same principle—a thin 
membrane separating the blood from the atmosphere. 
(1) Protozoa, Sponges and Polyps have no separate respir¬ 
atory apparatus, but absorb air, as well as food, from the 
currents of 'water passing through them or bathing the 
surface of their bodies. 
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 aerated 
water, used for locomotion as well as respiration. This 
may be called the “ water-pipe system.” Besides this, 
