Source : Webster's Revised Unabridged Dictionary (1913)

Bladder \Blad"der\, n. [OE. bladder, bleddre, AS. bl?dre,
   bl?ddre; akin to Icel. bla?ra, SW. bl["a]ddra, Dan. bl[ae]re,
   D. blaar, OHG. bl[=a]tara the bladder in the body of animals,
   G. blatter blister, bustule; all fr. the same root as AS.
   bl[=a]wan, E. blow, to puff. See {Blow} to puff.]
   1. (Anat.) A bag or sac in animals, which serves as the
      receptacle of some fluid; as, the urinary bladder; the
      gall bladder; -- applied especially to the urinary
      bladder, either within the animal, or when taken out and
      inflated with air.

   2. Any vesicle or blister, especially if filled with air, or
      a thin, watery fluid.

   3. (Bot.) A distended, membranaceous pericarp.

   4. Anything inflated, empty, or unsound. ``To swim with
      bladders of philosophy.'' --Rochester.

   {Bladder nut}, or {Bladder tree} (Bot.), a genus of plants
      ({Staphylea}) with bladderlike seed pods.

   {Bladder pod} (Bot.), a genus of low herbs ({Vesicaria}) with
      inflated seed pods.

   {Bladdor senna} (Bot.), a genus of shrubs ({Colutea}), with
      membranaceous, inflated pods.

   {Bladder worm} (Zo["o]l.), the larva of any species of
      tapeworm ({T[ae]nia}), found in the flesh or other parts
      of animals. See {Measle}, {Cysticercus}.

   {Bladder wrack} (Bot.), the common black rock weed of the
      seacoast ({Fucus nodosus} and {F. vesiculosus}) -- called
      also {bladder tangle}. See {Wrack}.

Source : WordNet®

bladder worm
     n : encysted saclike larva of the tapeworm