THE BUILDING-STONES OF THE ORGANIC WORLD 77 



dies after having been entirely separated from protoplasm, whilst 

 it was able to reconstruct another complete animal as long as 

 only a trace of protoplasm had been left adhering to it. We are 

 therefore able to say, by way of summing up, that the functions 

 of the cell-body are chiefly locomotion and the absorption of food, 

 whilst the nucleus controls the metabolism and regulates most 

 of the vital functions of the cell. Only as long as both parts col- 

 laborate will the life-machine be able to do its work. 



However simple the structure of the cell may appear from 

 the foregoing remarks, it can nevertheless assume the most mani- 

 fold forms. Just as the body of a higher organism is divided into 

 a great number of single organs and tissues, and as each of these 

 parts performs a definite vital task, so we observe in the single 

 cell a far-reaching division of labour. This is most striking in 

 the unicellular protozoa in which the one cell, depending upon 

 itself alone, must minister to all the wants of life. We find, 

 therefore, that in them the protoplasm frequently proceeds to 

 the formation of certain minute organs of motion, organella, 

 as we will call the different instruments of the single cell, as 

 distinguished from the multicellular ' organs ' of the metazoans 

 and metaphytes. We are able principally to distinguish two 

 types : thin delicate hairs or cilia, which are mostly distributed 

 in immense numbers along the surface of the body, or a few 

 long, whip-like forms, or flagella. By means of whipping 

 movements these cilia or flagella enable the cell to glide easily 

 through the water. In numerous protozoa we find, further, 

 that the cell-body is divided into several sharply defined layers, 

 each of which performs a separate task. The external plasm 

 is capable of becoming rigid, thus forming a protecting cover ; 

 another part forms special weapons of defence, and only the 

 inner protoplasm containing the nucleus continues to perform 

 its original task of assimilation. We even know numerous 

 unicellular protozoa which have proceeded to the formation of 

 a cell-mouth and cell-anus, possess a complex system of excretion, 

 and even simple organella of sight and touch. Like the protozoa, 

 the body-cells of the higher animals may be differently con- 

 structed, corresponding to their special functions. An ovum 

 differs in appearance from a spermatozoon, a nerve-cell from an 

 epithelial cell. A glance at the illustration (p. 78) will show 

 these radical differences. 



