When the socially joined epidermic cells at the surface of the tissue-body thrust forth in common a protective covering, we get the cuticles, which are often thick and solid armor-plates. In many of the metaphyta wax and flinty matter are deposited in the cellulose cuticles. The strongest formation is found in the invertebrate animals, where the cuticle often determines the whole shape and articulation, as in the calcareous shells of mollusks (mussel-shells, snail-shells, cockle-shells, etc.); and especially the coats of the articulata (the crab's coat of mail, and the skins of spiders and insects). VII UNITIES OF LIFE Units of life--Simple and complex organisms--Morphological and physiological individuals--Morphonta and bionta--Stages of individuality: cell, person, stem--Actual and virtual bionta--Partial and genealogical bionta--Metaphysical individuals--Cells (elementary organisms)--Cell membranes--Unnucleated cells--Plastids (cytodes and cells)--Primitive cells and nucleated cells--Organella (cell organs)--Cell communities (cœnobia)--Tissues of histona--Systems of organs--Organic apparatus--Histonal individuals (sprouts and persons)--Articulation of the histona (metamerism)--Stems of the histona--Animal states. The dissection of the body of the higher animal and plant into its various organs soon prompted comparative anatomists to draw a distinction between simple and complex organisms. Then, when the cell-theory developed in the course of the last half-century, the common anatomic groundwork of all living forms was recognized in the cell; and the conception of the cell as the elementary organism led to the further belief that our own frame, like that of all the higher animals and plants, is a cell-state, composed of millions of microscopic citizens, the individual cells, which work more or less independently therein, and co-operate for the common purposes of the entire community. This fundamental principle of the modern cell-theory was applied with great success by Rudolph Virchow to the diseased organism, and led to most important reforms in medicine. The cells are, in his view, independent "life-unities or individual life-centres," and the unified life of the whole man is the combined result of the work of his component cells. In this way the cells are the real life-unities of the organism. Their individual independence is at once seen in the permanently unicellular protists, of which several thousand species are already known to us.