The Cells of the Body 7 



of cells all looking alike, and the whole some- 

 what resembling a mulberry in shape. But 

 now a change comes; the cells on the outside 

 become longer than the rest, and little hair- 

 like processes, called cilia, grow out from them 

 and begin to vibrate to and fro, and, acting 

 like tiny oars, propel the little creature through 

 the sea. Presently the rest of the cells arrange 

 themselves so as to form a central cavity, with 

 an opening at one side, the whole looking like 

 a tiny cup. The animal now attaches itself to 

 a sea-weed or a rock, and no longer needing 

 the locomotive cilia, they disappear; but as it 

 can no longer travel, it can no longer seek its 

 food, which must be brought to it. Accordingly 

 we presently find that through the sides of its 

 body little holes appear, and the cells lining 

 the central cavity lengthen and develop cilia, 

 whose vibrations maintain a current of water 

 through the body, which brings with it oxygen 

 and food. This is the adult olynthus. 



Now observe, if you please, what has hap- 

 pened in the development of this little crea- 

 ture. A single cell divided into several cells, 

 at first all just alike, and all doing the same 



