pilidium

from The Century Dictionary.

• noun A generic name given to the larvæ of rhynchocœle tur-bellarians, or nemertean worms, under the impression that they are distinct animals.
• noun In conchology, a genus of false limpets of the family Acmæidæ.
• noun [lowercase] In botany, a hemispherical apothecium in certain lichens.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Zoöl.) 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.