22 THE FUNCTIONS OF ANIMALS. 
Division of labour.—All the ordinary functions of life 
are exhibited by the simple unicellular animals or Protozoa. 
Thus the Amceba 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 soon. 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 athird. And this division of labour is associated with 
that complication of structure which we call differentiation. 
Thus in the fresh-water Aydra, 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, Phénomenes de la 
vie communs aux animaux et aux végétaus, The beech- 
tree feeds and grows, digests and breathes, as really as does 
