The Collaborative International Dictionary of English v.0.48:

Pilidium \Pi*lid"i*um\, n.; pl. Pildia. [NL., fr. Gr. ?, dim.
   of ? a cap.] (Zool.)
   The free-swimming, hat-shaped larva of certain nemertean
   worms. It has no resemblance to its parent, and the young
   worm develops in its interior.
   [1913 Webster]