epizoic

American Heritage® Dictionary of the English Language, Fourth Edition

1. adj. Living or growing on the external surface of an animal.

Century Dictionary and Cyclopedia

1. 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.
2. Specifically, of or pertaining to the crustaceous parasites known as Epizoa.
3. Also epizoal, epizoan.

Wiktionary

1. adj. living or growing on the external surface of an animal (not always as a parasite)

GNU Webster's 1913

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

WordNet 3.0

1. adj. living or growing on the exterior surface of an animal usually as a parasite