PHYSIOLOGY. 401 



We have seen that the fluid propelled by the heart may 

 have 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 excretory organs. The fluid which does all 

 this is the blood. 



It is further to be noted that the flow of the blood, un- 

 like that of the air in the tracheae of insects or the lungs 

 of vertebrates, is not tidal, but forms a complete circula- 

 tion, entering the heart by different vessels and different 

 openings than those by which it leaves that organ. 



There are other aspects 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 

 without come 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 most parts of this 

 nervous system are on the surface, but as this superficial 

 position is dangerous to such an important structure, we 

 find in all the higher animals that the nerve centres or 

 ganglia become removed to a deeper position, which 

 necessitates the development of nerve-cords to connect 

 them with the sensory system and with the muscles and 

 other parts. It is interesting that in all animals, even in 

 man, no matter how deeply situated or how thoroughly 



