The Collaborative International Dictionary of English v.0.48:

Sphaeridium \Sphae*rid"i*um\, n.; pl. Sphaeridia. [NL., fr.
   Gr. ??? a sphere.] (Zool.)
   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.
   [1913 Webster]