Word Study
ambulacrum
CIDE DICTIONARY
ambulacrum, n. [L., an alley or covered way.].
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. [1913 Webster]
For further exploring for "ambulacrum" in Webster Dictionary Online