204 THE METAZOA — DIVISION OF LABOR 



As we have seen in plants, this results in a large number of 

 collections of cells in the body, each collection alike in structure and 

 performing the same function. Such a collection of cells we call 

 a tissue. (See Chapter III.) 



Frequently several tissues have certain functions to peiform in 

 conjunction with one another. The arm of the human body 

 performs movement. To do this, several tissues, as muscles, 

 nerves, and bones, must act together. A collection of tissues per- 

 forming certain work is called an organ. 



In the sponge, division of labor occurs between the cells of the 

 simple animal, some cells lining the incurrent pores creating a 

 current of water, and feeding upon the minute organisms which 

 come within reach, other cells building the skeleton of the sponge, 

 still others becoming eggs or sperms. Division of labor of a more 

 complicated sort is seen in the hydra. Here the cells which do the 

 same kind of work are collected into tissues, each tissue being a 

 collection of cells, all of which are more or less alike and do the 

 same kind of work. But in higher animals which are more 

 complicated in structure and in which the tissues are found work- 

 ing together to form organs, division of labor is still more devel- 

 oped. In the human arm, an organ fitted for certain movements, 

 think of the number of tissues and the complicated actions which 

 are possible. The most extreme division of labor is seen in the 

 organism which has the most complex actions to perform and 

 whose organs are fitted for such work. 



In our daily life in a town or city we see division of labor between 

 individuals. Such division of labor may occur among other ani- 

 mals, as, for example, bees or ants. But it is seen at its highest 

 in a great city or in a large business or industry. In the stockyards 

 of Chicago, division of labor has resulted in certain men performing 

 but a single movement during their entire day's work, but this 

 movement repeated so many times in a day has resulted in wonder- 

 ful accuracy and increased speed. Thus division of labor obtains 

 its end. 



Tissues in the Human Body. — Every animal body above the 

 protozoan is composed of a certain number of tissues. The cells 

 making up these tissues have certain well-defined characteristics. 

 In very simple animals the cells are all very much alike, but in more 



