The arthropods include crustaceans , insects , centipedes , millipedes , symphylans, pauropodans, and the extinct trilobites . Because the jointed exoskeleton blocks growth of the organism, it must be shed periodically.
Arthropods are mainly terrestrial, but aquatic representatives are well known.
There are three subphyla, comprising nine classes. The trilobites comprise a wholly extinct group of primitive marine arthropods. They were extremely abundant in the Cambrian and Ordovician geologic periods, becoming extinct in the Permian. Most of the 3,900 species ranged in length from 1 to 4 in. Triarthrus eatoni was a fossil trilobite common in the Ordovician seas.