sphaeridium
Sphæ*rid"i*um, n.; pl. Sphæridia. Etym: [NL., fr. Gr. (Zoöl.) Defn: A peculiar sense organ found upon the exterior of most kinds of sea urchins, and consisting of an oval or sherical head surmounting a short pedicel. It is generally supposed to be an olfactory organ.