194 SCIENCE FROM AN EASY CHAIR 



36A) is so called because of its constant change of 

 shape; it is also called Amceba on this account. It 

 flows out into broad, sometimes elongated, finger-like 

 processes, of which one or several of different sizes may 

 be formed at the same time, and then quickly disappear 

 as the whole creature moves. Solid particles of food 

 minute unicellular plants are engulfed by the moving 

 viscid protoplasm and digested within it that is to say, 

 chemically dissolved, just as food is digested in the 

 stomach of a big animal. The colourless cells of our 

 blood and lymph (Fig. 366) are called "amoeboid," 

 because of their identity with an Amceba in shape and 

 movement and digestive power. In some of these 

 animalcules (sun-animalcules and others) the processes 

 of the protoplasm are in the form of very fine, long 

 spreading threads which entangle a food particle, and 

 then contract, drawing it up into the disc-like central body. 

 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 com- 

 pared. 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. 4OA 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 



