This  community  is  expressed  by  placing  them  in  a  group  by  themselves
— the  Rhizopoda — separated  from  the  Actinopoda  in  which  the  thin
tapering  pseudopodia  radiate  stiffly  from  the  body  all  round.  Of  the
first  section  of  Actinopoda — the  HELIOZOA — -we  will  consider  first  Actino-
sphaerium  which  is  common  in  freshwater  pools  and  excellently  suited
for  laboratory  study.
ACTINOSPHAERIUM

A  well-developed  Actinosphaerium  is  clearly  visible  to  the  naked
eye  as  a  round  greyish-white  spot  when  seen  against  a  dark  background.
Under  a  low  magnification  it  is  seen  to  float  freely  in  the  water,  spherical
in  form  with  stiff  pseudopodia  radiating  out  all  round  making  it  resemble
the  conventional  figure  of  the  sun  and  justifying  its  popular  name  "  Sun
animalcule."  As  in  the  floating  types  of  Foraminifera  the  cytoplasm
(Fig.  u)  is  highly  vacuolated,  there  being  a  superficial  layer  of  vacuoles
of  specially  large  size  marking  the  ectoplasm,  while  in  the  endoplasm  the
vacuoles  are  much  smaller  so  that  it  has  a  more  opaque  appearance
than  the  ectoplasm.  Certain  of  the  vacuoles  of  the  ectoplasm — as  many
as  15  there  may  be  in  a  large  specimen — are  contractile  and  when  at
the  height  of  distension  (diastole)  they  may  be  observed  to  bulge
conspicuously  beyond  the  general  surface  (Fig.  n,  c.v).
