A whole group or division of these simplest animals are provided with special moving or vibrating hair-like extensions of the protoplasm called “cilia,” that being the Latin name for “eyelashes,” to which they are compared. These cilia are arranged with great regularity in rows, circlets, or spirals, on the surface of the “cell.” They are found not only on cells which are independent unicellular animals and plants, but also on cells which form the clothing or surface layer of many larger animals (Fig. 40A and B). Thus, in ourselves, they are found lining the windpipe, and they also line the internal cavity of the brain and spinal cord. The gills of the oyster, and such shell-fish, and other parts of their skin, are paved with ciliated or cilia-bearing cells, set side by side in thousands. A single “cilium” is like a little lash of a whip, and is always making its lashing movement. For a fraction of a second it is straight and upright, then suddenly curves over and bends to one side with a “flick,” and immediately recovers its upright position (see Fig. 29, p. 131). All the cilia on one cell or one surface “beat” in the same direction, and with a common rhythm, so that if the cell is a free, independent animalcule it is driven along through the water by the rapid strokes of these numberless tiny “oars,” or “paddles.” If the cilia are on a surface—like the oyster’s gill—they drive the water along and create a constant current. Each cilium consists of an elastic and a contractile fibre closely fused together: the contraction of the one part causes a flick or bending of the hair-like cilium, the elasticity of the other substance causes it at once to straighten out again.