104 PLANTS, THE FOOD OF ANIMALS 



We have seen that Paramecium and perhaps the majority 

 of protozoa live on bacteria, extracting from these minute cells 

 the elements necessary for their protoplasmic reconstruction 

 and growth. Rotifers and Crustacea feed upon the larvae 

 of animals, on smaller Crustacea and rotifers, and upon uni- 

 cellular animals and plants such as protozoa, diatoms, desmids 

 and other algae. These Crustacea, rotifers and larvae live 

 on unicellular animals and plants. The food of Hydra, there- 

 fore, traced back upon any line, finally brings us to the 

 minute chlorophyll-bearing plants and bacteria. These ulti- 

 mate food materials of Hydra, therefore, are living things some 

 of which have the power to manufacture their nutriment from 

 simple elements with the aid of the sun, while the others live 

 upon dissolved protein matters from decomposing animal and 

 plant tissues, or else utilize waste matters like urea and rela- 

 tively simple chemical compounds for their sources of energy. 



The higher animals, like Hydra, must likewise turn to the 

 plant world for food, but to trace back the connection in some 

 cases would involve a far more complicated chain of organisms 

 than in the case of the simple coelenterate. Man is almost 

 omnivorous, taking his food directly from the vegetable king- 

 dom in his green vegetables, cereals, etc., and from the animal in 

 his beef, mutton, fish, or fowl. Cattle, sheep and birds, in turn, 

 are herbivorous or graminivorous and get their main nourish- 

 ment from plants. Birds, indeed, eat worms and insects to a 

 great extent but worms and insects feed upon leaves and other 

 vegetable matter, so this food is only one step removed from 

 green matter. Man eats fish, oysters and other marine food; 

 the larger fish eat smaller ones, these still smaller and so on until 

 the smallest eat Crustacea, larvae, protozoa and other micro- 

 organisms which, like rotifers, subsist finally on microscopic 

 algae and bacteria. Carnivorous animals live almost entirely 

 without green plants or their products, and with them the 

 ultimate plant-eating forms serving as food are still more remote; 

 in the end, however, we find the same dependence upon the 

 vegetable world for proteins and potential energy. 



The work of Hydra and higher animals is done at the expense 



