THE EARTHWORM 205 



one of the many kinds of sugar in nature, are frequently made 

 from proteids by the protoplasm in the cells, and are stored 

 in the liver-cells of the higher animals, to be later transported 

 to their muscle-cells and stored in them until needed. In 

 animals that have no liver, as the earthworm, the glycogen 

 is stored in the cells of the lining of the body-cavity, and in 

 the muscles. A French physiologist, Chauveau, believes that 

 "the glycogen incorporated in the muscular tissue puts at 

 the service of the tissue the energy which it needs for its 

 work." This theory is valuable, because it enables us to see 

 how the energy (heat) released by the oxidation of the small 

 particles of glycogen in the muscle-cells may be transformed 

 into muscular energy without loss. Fats, made either from 

 carbohydrates or from proteid food by the protoplasm, or 

 stored directly in cells from the fatty acids absorbed through 

 the intestine, are reserve material, and are capable of supply- 

 ing energy when there is need of it. 



Second Stage of Respiration. When carbohydrates and fats 

 are oxidized the resulting compounds are carbon dioxide and 

 water, as these foods contain only carbon, hydrogen, and 

 oxygen. Proteids are far more complex. All proteids con- 

 tain at least carbon, hydrogen, nitrogen, oxygen, sulphur, and 

 usually, in addition, phosphorus. When they are oxidized 

 many different compounds result. The best known of these 

 are uric acid, carbon dioxide, and water. All these com- 

 pounds are wastes. Carbon dioxide, whether derived from 

 carbohydrates, fats, or proteids, makes its way by the blood 

 to the skin-capillaries of the earthworm, and there passes 

 through the moist membrane to the air outside. The first 

 stage of respiration consists of the absorption of oxygen by 

 the blood-vessels of the skin ; the second stage is the excre- 

 tion of carbon dioxide from the same blood-vessels, through 

 the skin. The entire process of respiration consists simply 

 of an exchange of gases through a membrane. 



