epizoic

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

• adjective Living or growing on the external surface of an animal.

from The Century Dictionary.

• In natural history, living on the surface or in the skin of animals, as lice, ticks, and many other insects, various parasitic fungi, etc. Also epizoötic.
• Specifically, of or pertaining to the crustaceous parasites known as Epizoa.
• Also epizoal, epizoan.

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

• adjective (Zoöl.) Living upon the exterior of another animal; ectozoic; -- said of external parasites.

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

• adjective living or growing on the external surface of an animal (not always as a parasite)

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

• adjective living or growing on the exterior surface of an animal usually as a parasite