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]

The Collaborative International Dictionary of English v.0.48:

Cysticerce \Cys"ti*cerce\ (s?s"t?-s?rs), Cysticercus
\Cys`ti*cer"cus\ (-s?r"k?s), n. [NL. cysticercus, fr. Gr. ????
   bladder + ke`rkos tail: cf. F. cysticerque.] (Zool.)
   The larval form of a tapeworm, having the head and neck of a
   tapeworm attached to a saclike body filled with fluid; --
   called also bladder worm, hydatid, and measle (as, pork
   measle).
   [1913 Webster]

   Note: These larvae live in the tissues of various living
         animals, and, when swallowed by a suitable carnivorous
         animal, develop into adult tapeworms in the intestine.
         See Measles, 4, Tapeworm.
         [1913 Webster]

WordNet (r) 3.0 (2006):

hydatid
    n 1: cyst filled with liquid; forms as a result of infestation
         by tapeworm larvae (as in echinococcosis)