20 THE FUNCTIONS OF ANIMALS. 



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 Amoeba 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 masses or cells. It 

 is impossible for these to remain the same, for some are 

 internal and others external, nor would it be well for the 

 organism that all its units should retain the primitive and 

 many-sided qualities of Amoebae. Division of labour, con- 

 sequent 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, Ph'enomenes de la 

 vie communs oux animaux et aux vegetaux. The beech- 

 tree feeds and grows, digests and breathes, as really as does 



