An order of
Entomostraca possessing shrimplike bodies enclosed in hard bivalve shells; called also
seed shrimp and
mussel shrimp. They were formerly referred to as
Ostracoidea. They are of small size, and swim freely about; many are less than 1 mm in length. Microfossils of certain extinct orders have been found extending back to the Cambrian age. [
1913 Webster]