The Collaborative International Dictionary of English v.0.48:

Ecthoreum \Ec`tho*re"um\, n.; pl. Ecthorea. [NL., fr. Gr. ? to
   leap out; ek out + ?, ?, to leap, dart.] (Zo["o]l.)
   The slender, hollow thread of a nettling cell or cnida. See
   Nettling cell. [Written also ecthor[ae]um.]
   [1913 Webster]