Internal   exchange  surfaces  of
   complex animals.
<P>
   Many  animals  have  organ  systems  with  finely  branched  or folded
   surfaces specialized for exchange.
<P>
<lI> Digestive  system:  the  small  intestine is lined with fingerlike
projections for nutrient absorption.
<lI> Respiratory system: the spongelike surface of the lung anables gas
exchange.
<lI> Excretory system: a kidney is composed of microscopic tubules that
exhange chemicals with blood flowing through tiny capillaries.