Webster’s Revised Unabridged Dictionary · 1913
Sphæridium
Sphæ·rid′i·um (sfē·rĭd′ĭ·ŭm), n.; pl. Sphæridia (#). [NL., fr. Gr. σφαῖρα a sphere.] (Zoöl.) 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.