SphaeridiumSph[ae]*rid"i*um\, n.; pl. Sph[ae]ridia. [NL., fr. Gr. ??? a sphere.] (Zo["o]l.) 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.