The Collaborative International Dictionary of English v.0.48:

Aplysia \A*plys"i*a\, n. [Gr. ? a dirty sponge, fr. ? unwashed;
   'a priv. + ? to wash.] (Zool.)
   A genus of marine mollusks of the order Tectibranchiata;
   the sea hare. Some of the species when disturbed throw out a
   deep purple liquor, which colors the water to some distance.
   See Illust. in Appendix.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Aplysia
    n 1: type genus of the family Aplysiidae [syn: Aplysia, genus
         Aplysia, Tethys, genus Tethus]