CELLS AND THEIR ASSOCIATION 43 



and waste go on between organisms and their environ- 

 ment. The processes involved, secretion and absorp- 

 tion, cannot be reproduced with non-living membranes. 



Free-living Cells. A cell from the body of one of the 

 higher animals cannot continue to live when separated 

 from its fellows. But nature abounds in organisms 

 which are single cells. These may be reckoned accord- 

 ing to their nutritional requirements in different instances 

 as either plants or animals. So the bacteria are con- 

 sidered to be plants because they can live on classes of 

 supplies which are simpler in their constitution than 

 those which animal life ordinarily demands. The 

 Infusoria of stagnant water, which are seen to devour 

 organic matter and whose lively habits constantly re- 

 mind us of the higher forms, are regarded as animals. 

 But, as has been stated before, the distinctions between 

 plants and animals have only a limited value when 

 applied to the lower orders of life. 



A free-living cell of the animal type has probably all 

 the activities which have been described in the preceding 

 chapter as characteristic of animals in general. That 

 is to say, it appropriates supplies of complex matter, 

 using some to promote its own growth but more to furnish 

 the energy for movement and heat production. This 

 energy it makes available by a respiratory process in 

 which oxygen is consumed and simple end-products 

 formed. Movement in the independent cell, as in the 

 larger organisms, is the expression of contraction. Some 

 explanation of this term is desirable. 



Contraction, as the biologist understands it, does not 

 mean diminution of volume. It is thus different from 

 contraction in the physical sense. When the mercury 

 in the thermometer tube contracts under the influence 

 of cold there is an actual reduction of the space which the 

 metal occupies. When a muscle contracts it can be 

 shown that the volume is unchanged; there has been a 

 shortening in one dimension but a compensating thick- 

 ening in others. Changes of form which are observed in 



