Nauplius

The oval body, with the abdomen not yet differentiated, and bearing no limbs; the (usually) three pairs of limbs, representing the first and second antennæ and mandibles of the adult, together with the single eye, are the distinguishing characteristics of the nauplius stage.

Here is an embryo, or young form, which has just escaped from the egg, and is hurrying off seawards to "see life," without doubt, and to begin life in earnest as well. You now behold a little body, which, roughly described, we shall say has a triangular shape. It has a tail behind, and the shield or "shell" with which the little body is covered, is prolonged in front and at its side-angles or corners into spines or horns. Three pairs of feet, or appendages that resemble these organs, are possessed by the infant barnacle, the two hinder pairs being forked at their tips and provided with long bristles. A single eye appears in front of the two foremost "feet;" and a mouth, stomach, and intestine are discovered within this little body. Thus provided within and without, this little Cyclopean creature swims merrily through the sea. In this stage it is universally named the Nauplius.

Fairy-shrimps are primitive crustaceans, most of which live in temporary ponds, roadside ditches, or marshes that dry up in sumer. They lay thick-walled eggs that withstand summer drying and winter freezing, lying dormant in the mud until they hatch out as naupliuses when wetted by the first heavy spring rain.

It is not certain that phaseluses are trilobites and not naupliuses of crustaceans or an extinct group of arthropods that has yet to be recognized.

The copepod nauplii concentrations were significantly different among stations and by depth (Appendices 3 and 4).

I suggest that several species of nauplioids hybridized with representatives of several groups of arthropods to give them nauplius larvae. […] The occurrence of nauplius larvae in crustacean taxa appears to be almost random, and it bears little relationship to adult morphology or to evolutionary history. This is consistent with the proposal that nauplii were acquired independently and at various times, and it challenge the widely held view that nauplii played a fundamental role in crustacean evolution.

Only nauplii that are attracted to the light at the top of the hatching tank should be collected, since these are the healthy ones.

The taxonomic distribution of nauplii within Crustacea raises questions, however, about the evolution of a nonfeeding but swimming nauplius from an embryonized nauplius. Most malacostracans lack a free nauplius stage. Euphausiaceans and dendrobranchiate shrimp, which hatch as nonfeeding nauplii, are the exceptions. Inferred phylogenies of malacostracans imply that either the ancestor lacked a free nauplius and free nauplii subsequently evolved again, or that egg nauplii evolved multiple times[…].