114 PRINCIPLES OF ANIMAL BIOLOGY 



compound which ties up very httle potential energy, is a product of these 

 oxidations, so that the amount of carbon dioxide which an animal pro- 

 duces is often taken as an indication of the quantity of energy it uses. 



Respiration. — How is all the oxygen for these oxidations obtained? 

 There is not enough of it in the substances to be oxidized. The common 

 carbohydrates contain only about half enough oxygen to oxidize their 

 own carbon, even if all their oxygen were available — which it is not — for 

 that purpose. Fats, the other main source of energy, have even less 

 oxygen than the carbohydrates. The oxygen must therefore be intro- 

 duced from external sources. For land animals that source is the air, 

 about one-fifth of which is oxygen. Aquatic animals of most kinds 

 secure the oxygen which is dissolved in the water about them. 



The obtaining of oxygen is included in the process known as respira- 

 tion. In small animals — unicellular and small multicellular ones — oxygen 

 is absorbed more or less directly by the cells that use it. In the larger 

 animals, those in which most of the cells are too far away from the surface 

 to rely on this simple diffusion, respiration is a double process. That is, 

 the oxygen must first be got into their bodies, a process known as external 

 respiration, and then be conveyed to the cells where it is ultimately used. 

 Its absorption by these cells, often far within the organism, is called 

 internal respiration. In the protozoa, external and internal respiration 

 are merged into a single process, to which neither name may be properly 

 applied. 



Whether an animal must have any special devices to carry on its 

 external respiration depends on its oxygen requirement in relation to its 

 surface. A large animal has much less surface relative to its volume than 

 a small one has ; hence, in general, the larger animals must have structures 

 which greatly increase their absorptive surfaces. Warm-blooded animals 

 consume much more oxygen than do cold-blooded ones, and active 

 animals much more than sluggish ones. Even as large an animal as the 

 earthworm, which is cold-blooded and not very active, is able to absorb 

 enough oxygen through its general surface. Many smaller animals, 

 however, because they are active, require some sort of respiratory organ 

 for their external respiration. 



Types of Respiratory System. — Probably the earliest external respira- 

 tory organs devclopcxl in animals were gills. These may be employed by 

 aquatic animals, and by aerial animals having some way of keeping them 

 moist, for oxygen cannot bo absorbed through dry surfaces. A gill, 

 like any other respiratory organ, must furnish a large surface, since the 

 amount of oxygen taken in increases with increase of surface. It may 

 consist of branching or treelike projections (Fig. 93), or of bunches of 

 fine tubes, or of clusters of flat plates, or of numerous ridges or fingerlike 

 projections, or of sievelike sheets through which water passes. In 



