[Illustration: Photograph of a living _vorticella_, showing the contractile stalk and the cilia around the mouth. Compare this figure with that of the paramoecium. Which cell shows greater division of labor?] As we look higher in the scale of life, we invariably find that certain parts of a plant or animal are set apart to do certain work, and only that work. Just as in a community of people, there are some men who do rough manual work, others who are skilled workmen, some who are shopkeepers, and still others who are professional men, so among plants and animals, wherever _collections_ of cells live together to form an organism, there is division of labor, some cells being fitted to do one kind of work, while others are fitted to do work of another sort. This is called physiological division of labor. [Illustration: Enlarged lengthwise section of the hydra, a very simple animal which shows slight division of labor. _ba_, base; _b_, bud; _m_, mouth; _ov_, ovary; _sp_, spermary.] [Illustration: 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_. [Illustration: 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 flagellae or large cilia (_fla_).]