l6 THE ANIMAL ORGANISM 



particular portion of the work of the whole. Such parts are called 

 organs. Thus animals may have sense organs, such as the eyes and 

 ears, for the reception of stimuli ; nervous organs, forming a 

 nervous system (usually provided with a central station such as 

 the brain), for the conduction of impulses set up by these and 

 other stimuli, to the organs which carry out the main part of 

 the reaction ; locomotive organs, such as legs and wings and fins, 

 to carry the body towards food or from danger ; organs of offence 

 and defence, such as teeth and claws, for procuring food and 

 resisting attack ; organs of digestion, such as the stomach and 

 bowels ; organs of circulation, such as the heart and blood vessels, 

 which distribute digested food, carry waste matters to the 

 excretory organs, such as the kidneys, and gases to and from 

 organs of respiration, such as lungs and gills, and transport 

 materials in general ; and organs of reproduction. An organ may 

 consist of subsidiary organs. Thus the leg is supported by skeletal 

 organs known as bones, moved by muscles, and served by blood 

 vessels and nerves. A complex of parts which work together is 

 known as an organism, and this name is often applied to animals 

 and to plants, for plants also are provided with organs, and are 

 alive. The provision of separate organs for particular functions 

 is called organisation or differentiation ; the assignment of par- 

 ticular functions to separate organs is called, by analogy with the 

 similar separation of functions in modern industry, the division 

 of physiological labour. This exists to a very various extent 

 among animals, and of two animals that which has the larger 

 number of different organs is said to be the more highly organised 

 or more highly differentiated, or simply the higher, though this 

 last word is also used in a slightly different sense in connection 

 with the theory of evolution (Chap. 30). The higher the organism, 

 the greater is its efficiency in coping with its surroundings, the 

 greater the vicissitudes in them which it can survive. There are 

 also great differences in form between the organs of animals of 

 the same grade of organisation. Thus a butterfly is as highly 

 organised as a fish, but its organs are utterly different in form. 

 The differences in structure between animals may correspond to 

 differences in their modes of life. Many animals which live in 

 water have, for instance, very different organs of locomotion 

 and respiration from those which live on land ; the sense organs 

 of an internal parasite are much less highly differentiated than 

 those of an animal which has to seek food and avoid enemies 



