Euarthropoda

Euarthropoda is a major clade within the phylum Arthropoda that comprises the crown group of living arthropods and their fossil relatives. It is defined as the group descended from the most recent common ancestor of all living arthropods, and it excludes the velvet worms (Onychophora), which lie outside Euarthropoda. In many classifications, Euarthropoda represents the true, or crown, arthropods.

Members of Euarthropoda share a segmented, bilaterally symmetric body plan with a hard, jointed exoskeleton made of chitin (often reinforced with mineralization). They typically have paired, jointed appendages on many segments and a ventral nervous system. The body is often organized into regions such as a head and trunk (tagmosis), and most species grow by molting their cuticle. Eyes are commonly compound, though sensory structures vary, and reproductive strategies range widely.

Within Euarthropoda are the major living lineages including Chelicerata (spiders, scorpions, mites, horseshoe crabs), Myriapoda (millipedes and centipedes), and the pancrustaceans (crustaceans and hexapods, which include insects). This diversity spans aquatic and terrestrial ecosystems and encompasses a wide range of life histories and morphologies.

The fossil record of euarthropods extends from the Cambrian, with many early forms showing the characteristic segmented exoskeleton and appendage organization. The clade has been central to questions about the evolution of segmentation, limbs, and tagmosis in arthropods and remains a focus of ongoing phylogenetic study. The name derives from Greek eu- “true” and arthron “joint.”