COMPARATIVE PHYSIOLOGY. 829 



The animal, besides, needs for its fuel substances known 

 to the chemist as nitrogenous food, and the combustion of 

 this produces, besides the carbon dioxide and water, nitrog- 

 enous waste, and this, in all of the higher animals, is gotten 

 rid of by means of organs which can be grouped under the 

 common name of kidneys. Here are to be placed not only 

 those structures specifically called kidneys in the foregoing 

 pages, but also the green gland of the crayfish, the Malpi- 

 ghian tubes of insects, the nephridia of the earthworm, and 

 the organ of Bojanus in the clam. Even the contractile 

 vacuole of the Protozoa is to be regarded as an organ for 

 the excretion of nitrogenous waste. 



We have seen that the fluid propelled by the heart has a 

 large series of different purposes to fulfil. It must carry 

 nourishment from the digestive tract to the different parts 

 of the body ; it has to carry oxygen from the gills and lungs 

 to these various structures, and to carry the carbon dioxide 

 and water produced by work to the same lungs and gills, 

 while the nitrogenous waste must be carried to the kidneys. 

 The fluid which does all this is the blood. 



There is another aspect of animal physiology to be re- 

 viewed. The animal needs to be aware of the presence of 

 food and of the proximity of things injurious, to it. This 

 implies the formation of a sensory system, and naturally this 

 system must be on the outside of the body, for from with- 

 out comes both food and danger. The knowledge of the 

 presence of good or of evil would be of little value to the 

 animal were it without ability to avail itself of this knowl- 

 edge. Hence this sensory system is connected with a 

 nervous system; which directs and controls the actions of 

 the animal. In the lower animals this nervous system is 

 on the surface, but as this superficial position is dangerous 

 to such an important structure, we find in all the higher 



