Nauplius of a Prawn
A nauplius (plural nauplii) is the first larva of animals classified as crustaceans (subphylum of Arthropoda). It consists of a head and a telson. The thorax and abdomen, characteristic of adult crustaceans, have not developed yet. A prominent characteristic of nauplii is that they have only one compound eye, which will divide in two in later stages. Nauplii have three pairs of cephalic appendages with which they swim; in the adult these become the antennules, the antennae, and the mandibles. The name nauplii properly refers to crustacean larvae that use appendages that stem from the head (antennules and antennae) as their main means of swimming.
Encyclopaedia Britannica (11th ed., vol. 7) (New York, NY: The Encyclopaedia Britannica Company, 1910)