\Sph[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.