Arthropoda
. Arthropod, animal with a hard, outer skeleton and a jointed body and limbs. Arthropods make up a phylum of invertebrates that includes insects , such as ants, beetles, and butterflies; crustaceans , such as lobsters, shrimps, and crabs; and arachnids , including scorpions, spiders, and ticks. In terms of sheer numbers and the variety of niches they fill, arthropods are the most successful animals on Earth. More than one million arthropod species have been identified—more than 20 times the number of known fish, amphibian, reptile, bird, and mammal species combined. This figure is considered a low estimate of the phylum's actual size because many arthropod species have yet to be discovered and documented. Some scientists suggest the number of arthropod species in tropical forests alone may approach six million to nine million. Traditionally, living arthropods are classified among three subphyla: the Chelicerata, Crustacea, and Unirama. Trilobites , a fourth subphylum, flourished