This completes our account of the bony skeleton. We shall finish the description of the supporting framework by a word about what may be called the connective tissue skeleton. The bony skeleton serves to support the body as a whole and to permit the muscles to do their work; the individual organs and the cells which make them up are held in place by sheets and bands of connective tissue. These are coarse and strong when their purpose is to support a large and heavy organ like the stomach; they become finer and finer as the parts to be supported become smaller, and when the individual cells are reached the connective tissue which surrounds them is almost inconceivably delicate. So completely does connective tissue permeate the whole body that it has been said that if everything else could be dissolved away, leaving only this tissue in place, there would still remain a model of the body, complete to the last detail. CHAPTER VII MOTION Our account of the body has now reached the point where we can take up in detail the special activities of the different kinds of cells. The first to be considered is motion, both because it is the familiar sign of life, as pointed out in the first chapter, and because it has so much to do with everything that enters into life. There are probably no animals that live out their entire lives without making any active motions, although some parasites, like the tapeworm, are stationary most of the time. There are a number of different ways in which movements are brought about. The very simplest animals, which consist of nothing but a bit of protoplasm, move by causing the protoplasm to flow bodily in one direction or another, a projection of part of the protoplasm being balanced by withdrawal of an equal part on the opposite side, and the whole mass progresses in the direction of the first projection. Next beyond this simplest means comes motion by tiny threads of protoplasm which project beyond the surface of the cell and whip back and forth. The stroke of these threads or _cilia_, as they are called, is stronger in one direction than in the other, so the effect of their beating is to propel the cell of which they are part in one direction through the water; or if they are on a surface which is stationary they set up a current in the water itself. This latter is the means by which oysters and similar animals which are anchored to the rocks get their food supplies. In some one-celled animals there are only one or two large cilia at one end; these beat back and forth, propelling the animal much as a fish swims.