sphaeridium
From ZooTerms (Dictionary of Invertebrate Zoology)
sphaeridium (noun; plural -idia; Greek diminutive sphaira, ball): (Echinodermata) Minute, spherical bodies covered by ciliated epidermis, usually lodged in pits in the test or almost completely enclosed; confined to the ambulacral areas around the mouth or scattered along the entire ambulacra; believed to be gravity receptors.
