The Nature of Animal Life. 15 



boundary forms. It is sufficient to state that unicellular 

 plants are spoken of as protophyta, and unicellular animals 

 as protozoa, the whole group of unicellular organisms being 

 classed together as protista. The animals whose bodies 

 are formed of many cells in which there is a differentiation 

 of structure and a specialization of function, are called 

 metazoa, and the multicellular plants metaphyta. The re- 

 lations of these groups may be thus expressed — 



Animals. Plants. 



Metazoa. Protozoa. Protophyta. Metaphyta. 



Protista. 



There are three matters with regard to the life-process 

 of animals and plants concerning which a few words must 

 be said. These are (1) their relation to food-stuffs ; (2) 

 their relation to the atmosphere ; (3) their relation to 

 energy, or the power of doing work. 



With regard to the first matter, that of food-relation, 

 the essential fact seems to be the dependence of animals 

 on plants. Plants can manufacture protoplasm out of its 

 constituents if presented to them in suitable inorganic 

 form scattered through earth and air and water. Hence 

 the peculiar features of their form, the branching and 

 spreading nature of those parts which are exposed to the 

 air, and the far-reaching ramifications of those parts 

 which are implanted in the earth. Hence, too, the flattened 

 leaves, with their large available surface. Animals are 

 unable to manufacture protoplasm in this way. They 

 are, sooner or later, dependent for food on plant-products. 

 It is true that the carnivora eat animal food, but the 

 animals they eat are directly or indirectly consumers of 

 vegetable products. Plants are nature's primary producers 

 of organic material. Animals utilize these products and 

 carry them to higher developments. 



In relation to the atmosphere, animals require a very 

 much larger quantity of oxygen than do plants. This, 

 during the respiratory process, combines with carbon so 



