planula

noun

plan·​u·​la ˈplan-yə-lə 

plural planulae ˈplan-yə-ˌlē 

-ˌlī

: the very young usually flattened oval or oblong free-swimming ciliated larva of a cnidarian

Word History

Etymology

New Latin, from Latin planus level, flat — more at floor

First Known Use

1835, in the meaning defined above