CHAPTER VII 

 ONE-CELLED AND MANY-CELLED ANIMALS 



Of the animals so far studied, Amoeba, Paramoecium, 

 Vorticella and their allies have the minute body composed of 

 but a single cell. The others, the frog, grasshopper and hydra, 

 have the body composed of many cells. This distinction of 

 one-celled and many-celled body has led to the classification 

 of all animals into two primary groups, the Protozoa, including 

 all those with one-celled body, and the Metazoa, all those 

 with a many-celled body. They are groups of very unequal 

 size, as of the 500,000 (approximated) known kinds of living 

 animals all but about 10,000 are Metazoa. But the distinction 

 between Protozoa and Metazoa is very important; it is indeed 

 one of the most fundamental in animal structure and classifica- 

 tion. For although many-celled animals are undoubtedly 

 derived by descent from one-celled ones, yet the group of single- 

 celled animals, the Protozoa, is much larger than we should 

 expect it to be if it were simply the beginning of the animal 

 scale. It is not only a beginning stage in animal evolution, 

 but it is an evolutionary line of its own. There is a great deal 

 of variety and complexity in the structure, physiology and 

 mode of development within the protozoan branch. All this 

 diversity has, however, to be limited to the differences possible 

 to a single cell. The moment animal evolution made the step 

 from independent single cell to mutually dependent many cells, 

 united for life, infinitely greater possibilities of diversity in 

 structure and function and life history were open. And the 

 extraordinary variety of animal life as it appears to us now in 

 the various groups of Metazoa, is the result of Nature's taking 

 advantage of these possibilities. 



But the step was not a sharp one, nor was the attainment of 

 present-day animal complexity and diversity brought about 



39 



