The Collaborative International Dictionary of English v.0.48:

Mysis \My"sis\, prop. n. [NL., fr. Gr. ? a closing of the lips
   or eyes.] (Zool.)
   A genus of small schizopod shrimps found both in fresh and
   salt water; the opossum shrimps. One species inhabits the
   Great Lakes of North America, and is largely eaten by the
   whitefish. The marine species form part of the food of right
   whales.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Mysis
    n 1: type genus of the family Mysidae [syn: Mysis, genus
         Mysis]