14 COMPARATIVE PHYSIOLOGY. 



CHAPTER II. 



COMPARATIVE PHYSIOLOGY. 



WE have seen that every animal organism exhibits 

 the four primary functions of alimentation, move- 

 ment, sensation and excretion. In the lowest types these 

 functions are performed indifferently by all parts of the 

 body, but in all the higher types we find that one part of the 

 body becomes specially concerned with one function, another 

 part with another function, and so on. In every case all 

 the functions are represented in the single organism and 

 each part becomes dependent on the others for the execution 

 of the other functions. The parts concerned with each 

 function are usually called systems and the subsidiary parts 

 of these systems are termed organs. The following systems 

 are connected with the primary functions : — 



Alimentation . . . ( i ) Alimentary system. 



Movement (2) Motor system (usually muscular system). 



Sensation (3) Sense-organs. 



Excretion I ^4) Respiratory system. 

 ■ " \ ( 5 ) Excretory system. 



Inter-communication between the various parts is established by the 

 (6) Nervotis syste7?i and (7) Circulatory system, whilst the function of 

 reproduction demands a separate (8) Rep7'oductivc system. Lastly, the 

 body is often supported and strengthened by the (9) Skeletal system. 



The principle of gradual relegation of certain functions 

 to certain parts of the body is termed physiological division 

 of labour and proceeds hand in hand, throughout the animal 

 kingdom, with growing complexity of structure. This 

 principle can be best understood by a simile. In a primi- 

 tive human community each man hunts for himself, each 

 builds his fire, makes the clothes and weapons he may 

 require, and so on. In more advanced communities, how- 

 ever, there occurs a division of labour. One man does 

 nothing but make weapons whilst another perhaps builds 

 houses, and each of these depends upon the rest of the 

 community for his other necessities. The result is in- 

 creased efficiency of the whole at the expense of the 



