Euarthropoda

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Euarthropod, or formally the Euarthropoda is a term that is often used to refer to the well-sclerotised arthropod clades, i.e., the chelicerates, pycnogonids, crustaceans, insects and myriapods. The need for the term arose in order to distinguish these well-known forms from their immediate relatives the onychophorans and tardigrades, which are also sometimes referred to as arthropods or panarthropods.

Some confusion about high-level arthropod taxonomy exists in the literature. Claus Nielsen coined the term "Panarthropoda" to encompass (eu)arthropods, onychophorans and tardigrades; in which case euarthropod is synonymous with arthropod, and thus redundant. However, other workers have pointed out that the prefix "Pan" in taxonomy properly refers to the total group of a clade, i.e., the stem group plus the crown group. If Panarthropoda is used in this way, then Arthropoda can be used to refer to these three clades, and Euarthropoda to the well-sclerotised taxa, with Tactopoda having been proposed to refer to Euarthropoda plus Tardigrada (if they are sister groups).