The Collaborative International Dictionary
Pilidium
Pilidium \Pi*lid"i*um\, n.; pl. Pildia. [NL., fr. Gr. ?, dim. of ? a cap.] (Zo["o]l.) 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.
Wiktionary
pilidium
n. (context zoology English) 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.