ANIMAL PHYSIOLOGY 61 



Protozoa. The simple digestive cavity of the hydra has 

 been referred to. In the polyps and jelly-fishes (see Chap- 

 ter XIII), this cavity is extended, the digestive surface 

 being much increased by partitions, tubes, etc. Worms, 

 crabs, and snails have a definite alimentary canal with certain 

 parts set apart for special processes. In the vertebrates 

 the digestive apparatus varies from a relatively simple 

 straight tube to the very long and complex alimentary canal 

 of the cow. All this variety depends much on the nature 

 of the food of the individual animal, and the processes 

 necessary to turn it into body material. 



We have now to consider that process which has to do 

 with carrying oxygen and food from the respiratory and 

 digestive surfaces to all parts of the body. This process 

 is the circulation, and the organs for performing it com- 

 pose the circulatory system. 



How the blood circulates. It has already been shown 

 that increase of size and activity in animals necessitates 

 blood and a means of circulating it through the body. The 

 uses of the circulation are: to bring oxygen from the respi- 

 ratory surface to every cell, to take carbon dioxide from 

 every cell to the respiratory surface, to carry digested food 

 substances from the absorbing surface of the alimentary 

 canal to every cell, and, further, to remove from every cell 

 the injurious and waste substances formed by its activity 

 to where they may be either excreted from the body or dis- 

 posed of in some other way. Circulation is accomplished 

 by the moving of a liquid through a system of tubes and 

 spaces channeling the whole body. 



In the very smallest and most sluggish of animals there is 

 no circulatory system. In those which are of compara- 

 tively large size and very active, and which therefore need 

 a great amount of energy, much oxygen and food must be 

 supplied. Also a large amount of waste substance is pro- 

 duced which must be removed. In such animals the cir- 



