noun Hy·drac·tin·ia \ˌhīˌdrakˈtinēə\

Definition of Hydractinia

  1. :  a genus of marine hydroids that have separate and distinctive polyps for nutritive, reproductive, and defensive functions borne on a dense encrusting coenosarc and that are commonly associated with shells containing hermit crabs


adjective or noun

Love words? You must — there are over 200,000 words in our free online dictionary, but you are looking for one that’s only in the Merriam-Webster Unabridged Dictionary.

Join MWU now and get access to America’s largest dictionary, with:

  • 300,000 words that aren't in our free dictionary
  • Expanded definitions, etymologies, and usage notes
  • Advanced search features
  • Ad free!

Origin and Etymology of hydractinia

New Latin, from hydr- + Actinia

Seen and Heard

What made you want to look up Hydractinia? Please tell us where you read or heard it (including the quote, if possible).


feeling or affected by lethargy

Get Word of the Day daily email!