The Collaborative International Dictionary of English v.0.48:

Hydatid \Hy"da*tid\, n. [Gr. ?, ?, a watery vesicle under the
   upper eyelid, fr. "y`dwr, "y`datos, water: cf. F. hydatide.]
   (Zool.)
   A membranous sac or bladder filled with a pellucid fluid,
   found in various parts of the bodies of animals, but
   unconnected with the tissues. It is usually formed by
   parasitic worms, esp. by larval tapeworms, as Echinococcus
   and C[oe]nurus. See these words in the Vocabulary.
   [1913 Webster]

   Hydatid of Morgagni (Anat.), one of the small pedunculated
      bodies found between the testicle and the head of the
      epididymis, and supposed to be a remnant of the
      M["u]llerian duct.
      [1913 Webster]