When  they  are  comparatively  short  and  thickly  crowded  on
the  surface,  or  a  part  of  the  surface,  of  the  cell,  they  are  called
cilia;  when  they  are  long  and  occur  singly  or  only  in  small
numbers  on  the  cell,  they  are  called  flagella.  They  occur  not
only  on  low  organisms,  like  bacteria  and  infusoria,  but  are  of
wide  distribution  and  great  importance  in  the  human  body;
e.  <?.,  where  cilia  occur  extensively  on  ciliated  epithelium,  and
flagella  on  the  spermatozoa.
The  motion  of  cilia  is  vibratory,  and,  as  they  act  in  unison,
a  wave-like  motion  results,  which,  acting  from  a  fixed  surface,
as  in  mammalia,  carries  particles  along  the  surface  or  creates
currents  in  the  fluids  adjacent.
The  motion  of  flagella  is  undulatory,  and  when  attached  to
free  cells  this  movement  rapidly  propels  the  cells  through  the
fluid  in  which  they  occur,  acting  in  the  same  manner  as  the
tail  of  a  tadpole  or  fish.  Flagella  therefore  are  a  means  for
rapid  locomotion  of  spermatozoa,  monads,  and  other  free-
swimming  cells  provided  with  them.
Other  cell- processes :  Besides  motile  processes  like  those  just
described,  some  cells  possess  permanent  and  non-motile  pro-
jections, which  may  be  very  various  in  form  and  size,  coarse
or  fine,  long  or  short,  branching  or  very  irregular.  Good  ex-
amples are  furnished  by  nerve-cells,  bone-cells,  connective-
tissue  cells.
Forms  of  cells :  Cells  vary  widely  in  shape,  according  to
the  situation  in  which  they  occur  and  the  purposes  they  serve.
