18 MANUAL OF ELEMENTARY ZOOLOGY 



of which does a particular portion of the work of the whole. 

 Such parts are called organs. Thus there are sense organs, 

 such as the eyes and ears, for the reception of stimuli; 

 nervous organs 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, to convey the digested food 

 about the body and to carry waste matters to the excretory 

 organs, such as the kidneys ; organs of respiration, such as 

 lungs and gills ; organs of reproduction, and so forth. 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 com- 

 plex of pa,rts which work together is known as an organism, 

 and this name is often applied to animals and to plants, 

 for plants are also provided with organs, and also alive. 

 The provision of separate organs for particular functions 

 is called organisation or differentiation; the assignment of 

 particular functions to separate organs which corresponds to 

 organisation is called, by analogy with the similar separation 

 of functions in modern industry, the physiological division of 

 labour. Organisation exists to a very various extent among 

 organisms, and of two organisms 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. 

 Thus man is a higher organism than a jelly-fish. There 

 are also great differences in form between the organs of 

 animals of the same grade of organisation. Thus an insect 

 is as highly organised as a fish, but its organs are utterly 

 different, in form. The differences in structure between 

 animals correspond to differences in their modes of life. 

 An animal which lives in water has, for instance, very 

 different organs of locomotion and respiration from one 

 which lives on land ; the sense organs of an internal para- 

 site are much less highly differentiated than those of an 

 animal which has to seek food and avoid enemies from hour 

 to hour; and a carnivorous animal has organs for seizing 



