Different forms of tissue cells. C, bone making cells; E, epithelial cells; F, fat cells; L, liver cells;M, muscle cell; i, involuntary; v, voluntary; N, nerve cell; C B, cell body; N.F., nerve fiber; T.B., nerve endings; W, colorless blood cells.
As we have seen, the higher plants are made up of a vast number of cells of many kinds. Collections of cells alike in structure and performing the same function we have called a tissue. Examples of animal tissues are the highly contractile cells set apart for movement, muscles; those which cover the body or line the inner parts of organs, the skin, or epithelium; the cells which form secretions or glands and the sensitive cells forming the nervous tissues.
Frequently several tissues have certain functions to perform 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 performing certain work we call an organ.
Part of a sponge, showing how cells perform division of labor. ect, ectoderm; mes, mesoderm; end, endoderm; c.c., ciliated cells, which take in food by means of their flagellæ or large cilia (fla).
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.