14 COMPARATIVE PHYSIOLOGY. 
CHAPTER II. 
COMPARATIVE PHYSIOLOGY. 
E 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...(1) Alimentary system. 
Movement...... (2) Alotor system (usually mezesceular system). 
Sensation ...... (3) Sezse-organs. 
(4) Respiratory system. 
Excretion ... (5) Zxcretory system. 
Inter-communication between the various parts is established by the 
(6) Nervous system and (7) Circulatory system, whilst the function of 
reproduction demands a separate (8) Reproductive 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 bya 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 
