sea slug

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. Any of various highly colorful marine gastropods of the suborder Nudibranchia, lacking a shell and gills but having fringelike projections that serve as respiratory organs. Also called nudibranch.

from Wiktionary, Creative Commons Attribution/Share-Alike License

• n. Any of several marine opisthobranch gastropod mollusks of the Nudibranchia

from the GNU version of the Collaborative International Dictionary of English

• A holothurian.
• A nudibranch mollusk.
• n. A holothurian.

from The Century Dictionary and Cyclopedia

• n. A marine opisthobran-chiate gastropod whose shell is rudimentary or wanting; a nudibranch, as a doridoid.
• n. A holothurian of any kind.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• n. any of various marine gastropods of the suborder Nudibranchia having a shell-less and often beautifully colored body