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 lowest animals, the 

 two are combined. But ia 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-hreathers 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 tliin 

 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 tlie 

 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. Tiiere 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, 



