Sentence:  It is not certain that phaseluses are trilobites and not <WSD>naupliuses</WSD> of crustaceans or an extinct group of arthropods that has yet to be recognized. 
Meaning of the word marked with <WSD> tags:  A crustacean larva that has three pairs of locomotive organs (corresponding to antennules, antennae, and mandibles), a median eye, and little or no segmentation of the body. (from mid 19th c.)