Daphnia

from WordNet (r) 3.0 (2006)

daphnia
    n 1: minute freshwater crustacean having a round body enclosed
         in a transparent shell; moves about like a flea by means of
         hairy branched antennae [syn: {daphnia}, {water flea}]

from The Collaborative International Dictionary of English v.0.48

Daphnia \Daph"ni*a\, n. [NL.] (Zool.)
   A genus of the genus {Daphnia}.
   [1913 Webster]