In a simple animal like a sponge, division of labor occurs between the cells; some cells which line the pores leading inward create a current of water, and feed upon the minute organisms which come within reach, other cells build the skeleton of the sponge, and still others become eggs or sperms. In higher animals more complicated in structure and in which the tissues are found working together to form organs, division of labor is much more highly specialized. 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, for there the cells or tissues which do the particular work do it quickly and very well. In our daily life in a town or city we see division of labor between individuals. Such division of labor may occur among other animals, 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 wonderful accuracy and speed. Thus division of labor obtains its end. Organs and Functions Common to All Animals.--The same general functions performed by a single cell are performed by a many-celled animal. But in the many-celled animals the various functions of the single cell are taken up by the organs. In a complex organism, like man, the organs and the functions they perform may be briefly given as follows:-- (1) The organs of _food taking_: food may be taken in by individual cells, as those lining the pores of the sponge, or definite parts of a food tube may be set apart for this purpose, as the mouth and parts which place food in the mouth. (2) The organs of _digestion_: the food tube and collections of cells which form the glands connected with it. The enzymes in the fluids secreted by the latter change the foods from a solid form (usually insoluble) to that of a _fluid_. Such fluid may then pass by osmosis, through the walls of the food tube into the blood. (3) The organs of _circulation_: the tubes through which the blood, bearing its organic foods and oxygen, reaches the tissues of the body. In simple animals, as the sponge and hydra, no such organs are needed, the fluid food passing from cell to cell by osmosis. (4) The organs of _respiration_: the organs in which the blood receives oxygen and gives up carbon dioxide. The outer layer of the body serves this purpose in very simple animals; gills or lungs are developed in more complex animals. (5) The organs of _excretion_: such as the kidneys and skin, which pass off nitrogenous and other waste matters from the body.