The Collaborative International Dictionary of English v.0.48:

Ectozoon \Ec`to*zo"["o]n\, n.; pl. Ectozoa. [NL., fr. Gr.
   'ekto`s outside outside + zw^,on an animal.] (Zo["o]l.)
   See Epizo["o]n.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ectozoon
    n 1: any external parasitic organism (as fleas) [syn:
         ectoparasite, ectozoan, ectozoon, epizoan,
         epizoon]