22 THE FUNCTIONS OF ANIMALS. 



of food, respiration, and excretion, besides the periodic 

 activities of growth and reproduction. 



Division of labour. All the ordinary functions of life 

 are exhibited by the simple unicellular animals or Protozoa. 

 Thus the Amoeba moves by contracting its living substance, 

 draws back sensitively from hurtful influences, engulfs and 

 digests food, gets rid of waste, and absorbs oxygen. 



But all these activities occur in the Amceba within the 

 compass of a unit mass of living matter, a single cell, 

 physiologically complete in itself. 



In all other animals, from Sponges onwards, there is a 

 " body " consisting of hundreds of unit areas or cells. A cell 

 is a unified area of living matter almost always with a definite 

 centre or nucleus. It is impossible for these cells to remain 

 the same, for as they increase in number they become 

 diversely related to the outer world, to food, to one another, 

 and so on. Division of labour, consequent on diversity of 

 conditions, is thus established in the organism. In some 

 cells one kind of activity predominates, in others a second, in 

 others a third. And this division of labour is associated with 

 that complication of structure which we call differentiation. 



Thus in the fresh-water Hydra, which is one of the 

 simplest many-celled animals, the units are arranged in 

 two layers, and form a tubular body. Those of the outer 

 layer are protective, nervous, and muscular ; those of the 

 inner layer absorb and digest the food, and are also muscular. 



In worms and higher organisms, there is a middle layer 

 in addition to the other two, and this middle layer becomes, 

 for instance, predominantly muscular. Moreover, the units 

 or cells are not only arranged in strands or tissues, each 

 with a predominant function, but become compacted into 

 well-defined parts or organs. None the less should we 

 remember that each cell remains a living unit, and that, in 

 addition to its principal activity, it usually retains others of 

 a subsidiary character. 



Plants and animals. Before we give a sketch of the 

 chief functions in a higher animal, let us briefly consider the 

 resemblances and differences between plants and animals. 



(a) Resemblance in function.- The life of plants is 

 essentially like that of animals, as has been recognised since 

 Claude Bernard wrote his famous book, Fhcnomenes de la 



