The Collaborative International Dictionary of English v.0.48:

Nauplius \Nau"pli*us\, n.; pl. Nauplii. [L., a kind of
   shellfish, fr. Gr. nay^s ship + ? to sail.] (Zool.)
   A crustacean larva having three pairs of locomotive organs
   (corresponding to the antennules, antennae, and mandibles), a
   median eye, and little or no segmentation of the body.
   [1913 Webster]