CHAPTER XIV 



THE SIMPLEST ANIMALS 



I. The Simplest Forms of Life — 2. Survey of Protozoa — 3. The com- 

 mon Amceba — 4. Structure of the Protozoa — 5. Life of Protozoa 

 — 6. Psychical Life of the Protozoa — 7. History of the Protozoa 

 8. Relation to the Earth — 9. Relation to other Forms of Life — 

 10. Relation to Man 



1 . The Simplest Forms of Life. — It is likely that the first breath 

 of life was in the water, for there most of the simplest animals and 

 plants have their haunts. Simple they are, as an egg is simple 

 when contrasted with a bird. They are (almost all) unit specks of 

 living matter, each comparable to, but often more complex than, one 

 of the numerous unit elements or cells which compose any higher 

 plant or animal, moss or oak-tree, sponge or man. It is not merely 

 because they are small that we cannot split them into separate parts 

 different from one another, — size has little to do with complexity, — 

 but rather because they are unit specks or single cells. But they are 

 not " structureless " ; in fact, old Ehrenberg, who described some of 

 them in 1838 as " perfect organisms " and fancied he saw stomachs, 

 vessels, hearts, and other organs within them, was nearer the truth 

 than those who reduce the Protozoa to the level of white of egg. 



Nor are they omnipresent, swarming in any drop of water. The 

 clear water of daily use will generally disappoint, or rather please 

 us by showing little trace of living things. But take a test-tube of 

 water from a stagnant pool, hold it between your eyes and the light, 

 and it is likely that you will see many forms of life. Simple plants 

 and simple animals are there, the former represented by threads, 

 ovals, and spheres in green, the latter by more mobile almost 

 colourless specks or whitish motes which dance in the water. But 

 besides these there are jerky swimmers whose appearance almost 

 suggests theirpopularname of "water-fleas," and wriggling "worms," 



