THE SIMPLEST LIVING THINGS 195 



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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. 



The ciliated unicellular animalcules (often called the 

 infusoria, because they flourish in decomposing "infusions") 

 not only swim by means of their cilia, but have a definite 

 mouth or opening in the firm outer layer of the proto- 

 plasm of the cell, into which solid particles of food are 

 driven by whirlpool-like currents set up by special lines 

 of cilia (Fig. 41 A a). The mouth leads through a definite 

 " gullet " into the interior of the cell. Remember that 

 the whole creature is but a single minute cell or corpuscle 

 of protoplasm ! It is only from the hundredth to the 

 thousandth of an inch long with nucleus (e in the figure) 

 of denser structure within just like, in essential structure 

 and properties, one single cell of the many thousands 

 which build up the liver, or are packed in layers to form 

 our outer skin, or are piled side by side (by self-division) 

 to make the stems and leaves of plants. Yet here is 

 such a cell self-sufficing. When it divides (as it does) 

 the two resulting cells do not remain in contact as they 

 do when a germ cell (a fertilised egg-cell) divides. They 

 simply separate, and each swims away, and carries on its 

 own life. Many of them are fitted out with these cilia 

 as a most serviceable locomotor apparatus, and as pro- 



