Contact  with  the  environment: simple
   animals.
<P>
<lI> In  a unicellular protist, such as this amoeba, the entire surface
area  contacts  the  environment,  and  material  can  efficiently
diffuse through the plasma membrane.
<lI> The  two  layers  of  cells in a hydra are also in directl contact
with  its  aqueous  environment  when water circulates through its
mouth.