100 RESPIRATION AND DISPOSAL OF WASTES 



sole organ of respiration. On the other hand, there are 

 many animals enclosed in shells or horny cases or scales, 

 feathers, or hair, as in the higher types, that are either 

 too thick or too dry to permit of the diffusion of gases 

 through the skin, and accessory respiratory organs, con- 

 structed on various plans, perform the duty of gas 

 exchange. 



Gills. — The conditions surrounding an animal that 

 spends its life in the water are very evidently different 

 from those of a land-dweller; hence it is not surprising 

 to find that the special respiratory organs of aquatic 

 forms are constructed on a different plan from those 

 breathing atmospheric air. They usually take the form 

 of delicate expansions or outgrowths of the skin known 

 as gills. In many of the worms they appear as great 

 plume-like structures borne on the head, or as feathery 

 or broad plate-like organs along the side of the body. 

 In some of the snails the gill resembles a many-petaled 

 flower carried on the back, or exists in the form of delicate 

 and slender outgrowths widely distributed over the body. 

 In the fishes they are comb-like organs situated along 

 the sides of the throat. In every instance these organs 

 are penetrated by an extensive capillary network or sys- 

 tem of blood spaces that are thus most favorably situated 

 for receiving oxygen and discharging carbon dioxid. Even 

 more favoraVjle conditions are present where the water 

 is kept in motion by movements of the gills, as happens 

 in many species of worms, shrimps, and some of the 

 salamanders, for example. 



Respiratory Organs of Land Animals. — In most 

 terrestrial animals the surface of the body is more or 

 less enclosed in a protective coat or outer skeleton that 

 reduces the chances of injury by enemies, heat, or light, 

 and at the same time prevents excessive evaporation of 

 water from the body. Furthermore, this investing sheath 

 is not only relatively thick but in most instances it is far 

 too dry to permit the ready exchange of gases through 



