62 THE LIVING WORLD. 



Fig. 2 shows a cell in the modern sense. All of the 

 living parts of animals and plants are composed of a 

 mass of such cells. The cells however, are far from 

 being all alike. Indeed, they differ extremely in 

 shape and size and in many other respects, but in all 

 cases each consists of a bit of nuclear matter sur- 

 rounded by protoplasm, and each is thus more or 

 less independent of the others. The larger and 

 higher animals are composed of a larger number of 

 cells, and always of a larger variety in their shape 

 and function. 



The cell has thus been for many years regarded as 

 the unit of life, and the life of the individual as the 

 sum of the life of its cells. When we go among the 

 lowest and simplest of animals, we find one large 

 group in which the individual consists of only a 

 single cell. These are the unicellular animals {Pro- 

 tozoa) and plants (Protophyta). They are all aquatic 

 organisms, all small, usually microscopic, all of course 

 extremely simple, but withal extremely abundant : 

 they are found in the ocean, in rivers, brooks, ponds, 

 pools, ditches, swamps, gutters, puddles ; in short, 

 wherever there is any water, there we may be pretty 

 sure to find some representatives of the unicellular 

 organisms. Fig. 3 represents a few types of these 

 animals. 



It was a great simplification of our knowledge of 

 animals and plants when it was discovered that all 

 were to be regarded as complexes of these simple 

 cells, for it gave a unit of life. The unicellular animals 

 and plants were naturally regarded as the simplest 

 possible organisms, since they consisted of one such 



