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.

Source: wiktionary

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.

Source: wiktionary

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.

Source: wiktionary

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.

Source: wiktionary