The Collaborative International Dictionary of English v.0.48:

Coenurus \C[oe]*nu"rus\, n. [NL. fr. Gr. koino`s common + o'yra`
   tail.] (Zool.)
   The larval stage of a tapeworm (T[ae]nia c[oe]nurus) which
   forms bladderlike sacs in the brain of sheep, causing the
   fatal disease known as water brain, vertigo, staggers or gid.
   [1913 Webster]

   Note: This bladder worm has on its surface numerous small
         heads, each of which, when swallowed by a dog, becomes
         a mature tapeworm in the dog's intestine.
         [1913 Webster]