THE METAZOA DIVISION OF LABOR 205 



complex animals the cells are more and more unlike as their 

 work becomes more and more different. Let us see what these cells 

 may be, what their structure is, and, in a general way, what func- 

 tion each has in the human body. 



Muscle Cells. A large part of our body is made up of 

 muscle. Muscle cells are elongated in shape, and have great con- 

 tractile power. Their work is that of causing movement, and this 

 is usually done by means of attachment to a skeleton inside the 

 body. In man they may be of two kinds, voluntary (under con- 

 trol of the will) and involuntary. 



Diagrams of sections of cells, greatly magnified, e, flat cell (epithelium) from 

 mouth ; c, columnar epithelium from food tube ; &, bone-forming cell ; I, liver 

 cell ; m, muscle cell ; /, fat cell ; n, nerve cell. 



Epithelial Cells. Such cells cover the outside of a body or 

 line the inside of the cavities in the body. The shape of such cells 

 varies from flat plates to little cubes or columns depending upon 

 their position inside or outside the body. Some bear cilia, an 

 adaptation. Can you think of their purpose ? 



Connective Tissue Cells. Such cells form the connection 

 between tissues in the body. They are characterized by possess- 

 ing numerous long processes. They also secrete, as do many 

 other cells, a substance like jelly, called intercellular substance. 

 This stands in the same relation to the cells as does mortar to the 

 bricks in a wall. 



Several other types of cells might be mentioned, as blood cells, 

 cartilage cells, bone cells, and nerve cells. A glance at the Figure 

 shows their great variety of shapes and sizes. 



