pilidium
/pʌɪˈlɪdɪəm/noun
Zoology. Also †Pilidium. The early, free-swimming, cap-shaped larva of nemertean worms of the order Heteronemertea (formerly thought to be a distinct genus).
Origin
Mid 19th century (in an earlier sense). Ultimately from ancient Greek πιλίδιον little cap from πῖλος felt cap + -ίδιον. In sense 1, via French pilidium. In sense 2, via scientific Latin Pilidium, genus name.