opisthobranch

noun

1. any gastropod mollusk of the order Opisthobranchia, as the sea slugs, sea butterflies, and sea hares, characterized by a vestigial or absent mantle and shell and two pairs of tentacles.

adjective

2. of or relating to the opisthobranches.

noun

1. any marine gastropod of the class Opisthobranchia (or Opisthobranchiata), in which the shell is reduced or absent: includes the pteropods, sea hares, and nudibranchs