Arthropoda (meaning "jointed feet") is a very diverse phylum of protostome invertebrates which contains all animals with a chitinous exoskeleton, branched and jointed legs, fused segments, and the ability to molt old skin. These animals are by far the most abundant and successful phylum since its evolution.
5 distinct subphyla has been described that fit under this clade, with one being completely extinct:
- Trilobita: consists of all trilobites, and all are currently extinct. Characterized by a flat body, the presence of 3 distinct body segments, and each segment has 2 legs.
- Chelicerata: named after their characterized appendages called chelicerae, the subphylum includes arachnids, horseshoe crabs, and sea spiders.
- Crustecea: includes all crustaceans, characterized by the presence of a thick carapace, 2 body segments, and multiple pairs of feeding appendages. Members of the clade are normally aquatic, but some live primarily on land.
- Myriapoda: made up of all centipedes and millipedes. They have a set of mandibles, one pair of antennae, a long segmented body, and a distinct head.
- Hexapoda: all insects make up this clade, and are characterized by the presence of 3 body segments, 3 pairs of legs, and a single pair of antennae. Insects are by far the most diverse and abundant of the 5 clades, and outnumber the rest 8 times.