>Ostracods are small crustaceans with the common name of water flea.  They
>are common freshwater plankton and a good indicator of water quality.  It is
>a shell (although not a calcite or aragonite as in mollusks) and they are
>free swimming with just feet and feelers sticking out of the shell and the
>rest of their body hidden and protected within.

Er... Daphnia (the "water flea") isn't an ostracod, though it bears
some resemblance.
--
Regards
Andrew