82 ELEMENTARY ZOOLOGY 



lifted above the water's surface. Similarly the rock 

 called Tripoli found in Sicily and the Barbadoes earth 

 from the island of Barbadoes are composed of the shells 

 of ancient Radiolaria. 



It is thus evident that the Protozoa is an ancient group 

 of animals. As a matter of fact zoologists are certain 

 that it is the most ancient of all animal groups. All of 

 the animals of the ocean depend upon the marine Protozoa 

 and the marine Protophyta, one-celled plants, for food. 

 Either they feed on them directly, or prey on animals 

 which in turn prey on these simplest organisms. A well- 

 known zoologist has said: "The food-supply of marine 

 animals consists of a few species of microscopic 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 dependence on it." The 

 marine Protozoa are the only animals which live in- 

 dependently; 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 is meant that their kind appeared earliest in the 

 history of the world, and as it is certain that ocean 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 all animal groups. 



As already learned in the examination of examples of 

 one-celled animals, it is evident that life may be success- 

 fully maintained without a complex body composed of 

 many organs performing their functions in a specialized 

 way. The marine Protozoa illustrate this fact admirably. 

 Despite their lack of special organs and their primi- 

 tive way of performing the life-processes, that they live 

 successfully is shown by their existence in such extraor- 

 dinary numbers. They outnumber all other animals. 



