A highly diverse phylum of jointed-limbed animals, which includes the crustaceans, arachnids, and insects as the major components, as well as the classes Symphyla, Pauropoda, Chilopoda (centipedes), Diplopoda (millipedes), and the extinct Trilobita and eurypterids (see chelicerata; merostomata). Arthropods first appeared in the Cambrian, already well diversified with such forms as the trilobites, trilobitoids, Ostracoda, and crabs present, implying an earlier, hidden history, reaching back into the Precambrian. Embryological evidence shows that they are derived either from primitive Polychaeta, Annelida, or from ancestors common to both. Arthropods share with annelid worms a metamerically segmented body (see metameric segmentation), at least in the embryo, a dorsal heart, a dorsal anterior brain, and a ventral nerve cord that has segmental, ganglionic swellings. The limbs of all arthropods are paired, jointed, and segmental, and the body has a chitinous exoskeleton. Primitively, the limbs and cuticular plates correspond to the metameric segmentation of the body, but in many groups there is considerable loss and/or fusion of segments.