20 ANIMAL LIFE 



as polishing powder, are composed almost exclusively of 

 the siliceous shells of ancient and long-extinct Radiolaria. 



10. Antiquity of the Protozoa, All the animals of the 

 ocean depend upon the marine Protozoa (and the marine 

 Protophyta, or one-celled plants) for food. Either they 

 prey upon these one-celled organisms directly, or they prey 

 upon animals which do prey on these simplest animals. 

 The great zoologist already quoted says : " The food sup- 

 ply of marine animals consists of a few species of micro- 

 scopic organisms which are inexhaustible and the only 

 source of food for all the inhabitants of the ocean. The 

 supply is primeval as well as inexhaustible, and all the life 

 of the ocean has gradually taken shape in direct depend- 

 ence upon it." That is, the marine simplest animals are 

 the only marine animals which live independently; they 

 alone can live or could have lived in earlier ages without 

 depending on other animals. They must therefore be the 

 oldest of marine animals. By oldest we mean that their 

 kind appeared earliest in the history of the world. As it 

 is certain that marine life is older than terrestrial life that 

 is, that the first animals lived in the ocean it is obvious 

 that the marine Protozoa are the most ancient of animals. 

 This is an important and interesting fact. Zoologists try 

 to find out the relationships and the degrees of antiquity 

 or modernness of the various kinds of animals. We have 

 seen that the Protozoa, those animals which have the sim- 

 plest body structure and perform the necessary life pro- 

 cesses in the simplest way, are the oldest, the first animals. 

 Tlrs is just what we would expect. 



11. The primitive form. We find among the simplest 

 animals a considerable variety of shape and some manifest 

 variation in habit. But the points of resemblance are far 

 more pronounced than the points of difference, and are of 

 fundamental importance. The composition of the body of 

 one cell, as opposed to the many-celled structure of the 

 bodies of all other animals, is the fact to be most distinctly 



