Chap. 13 THE RELEASE OF ENEROY^RESPIRATION 227 



the sun. Everybody sees the oxidation of dead cells in a burning cigarette, 

 with the energy escaping in light and heat. In chemical terms it is expressed as: 



C,iHi,.0« + 60, -^ 6CO, + 6H,0 + energy 



carbohydrate + oxygen yields carbon -f water -f energy 



dioxide 



Respiration may a^so occur without air, that is, anaerobically. When at- 

 mospheric oxygen is absent, oxidation is incomplete; only part of the energy 

 is released and certain intermediate compounds are formed. Anaerobic respi- 

 ration is a phase of the ordinary respiratory process rather than an entirely 

 different kind. It occurs in certain bacteria and in yeast cells. It is well known 

 and important in mammalian muscle. The ability of muscles to work for a 

 short time without oxygen is one of their most important characteristics (Chap. 

 10). This doubtless always occurs in athletic contests and in horse races. 



Arrangements for Respiration 



The simplest respiratory arrangements are in aquatic animals, usually small 

 ones (Fig. 13.3). The covering of these animals is thin and outspread. The 



Philodina 

 ROTIFER 



Stentor 

 PROTOZOAN 



Bosmina 

 CRUSTACEAN 



Nais 

 ANNELID WORM 



Fig. 13.3. Minute aquatic animals whose size and relatively large exposure of thin 

 membranes allow adequate diffusion and exchange of respiratory gases. 



bodies of protozoans, planarians, rotifers, and minute worms are thread- 

 shaped, branched, and star-pointed, with crevices and outriggers that welcome 

 oxygen. In all of them respiration is direct. Gases diffuse directly from water 

 into the cells and vice versa. Although they are much larger animals, sponges 

 and jellyfishes also depend upon direct respiration. They are able to do this 

 because they are extremely water-saturated, and their bodies are interlaced by 

 passageways through which circulating water distributes gases directly to and 

 from the cells. 



