sea slug

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun Any of various other marine gastropods that lack a shell or have a reduced shell.

from The Century Dictionary.

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

from the GNU version of the Collaborative International Dictionary of English.

• A holothurian.
• A nudibranch mollusk.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of several marine opisthobranch gastropod mollusks of the Nudibranchia

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

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