THE EARTHWORM 203 



been stored temporarily either in the lining of the body- 

 cavity, in the muscle-cells, or in the fat-cells. Protoplasm 

 itself breaks down, but to a less extent. 



First Stage of Respiration. The earthworm has neither gills, 

 tracheae, nor lungs; still it can breathe quite as perfectly as the 

 animals which possess one or another of those organs. The 

 essential characteristic of a breathing organ is a thin, moist 

 membrane, with thin-walled capillary blood-vessels on one side 

 and air on the other. The outer skin of the earthworm is 

 thin and moist, and just beneath it are capillaries. While in 

 its burrow, or even in water, air comes in contact with the 

 skin, and its most important element, oxygen, passes through 

 and mixes with the blood. The second stage of respiration 

 comes after oxidation, in the series of events here described. 



Oxidation. In the circulating blood there is a red-colored 

 Mibstance called haemoglobin. Oxygen combines chemically 

 with this, and the compound goes with the blood until it 

 reaches tissue-cells which have some food stored in them; 

 then the oxygen combines with the food, which may be car- 

 bohydrate, fat, or proteid. The chemical union of oxygen 

 with other elements is of the greatest importance in the life 

 of any plant or animal. A simple example of the result of 

 the chemical union of oxygen with another element may be 

 observed in the burning of coal. It is well known that coal 

 is composed chiefly of carbon. When a quantity of coal is 

 heated it begins to unite with oxygen from the air. During 

 this process four phenomena may be observed : first, the quan- 

 tity of oxygen in the room is reduced ; second, the quantity 

 of coal is reduced ; third, an invisible gas is formed ; and 

 fourth, heat and light are given off. 



When oxygen unites chemically with carbon two kinds of 

 gas may be formed, either carbon monoxide or carbon dioxide, 

 or both. The union of a solid element with a gaseous one 

 to form a gaseous compound accounts for the fact that so 



