The Collaborative International Dictionary of English v.0.48:

Ambulacrum \Am`bu*la"crum\, n.; pl. Ambulacra. [L., an alley
   or covered way.] (Zool.)
   (a) One of the radical zones of echinoderms, along which run
       the principal nerves, blood vessels, and water tubes.
       These zones usually bear rows of locomotive suckers or
       tentacles, which protrude from regular pores. In star
       fishes they occupy the grooves along the under side of
       the rays.
   (b) One of the suckers on the feet of mites.
       [1913 Webster]

WordNet (r) 3.0 (2006):

ambulacrum
    n 1: one of the five areas on the undersurface of an echinoderm
         on which the tube feet are located