Arthropoda

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See Arthropoda in Wikipedia, the free encyclopedia.

The arthropods constitute a phylum of among the most highly successful of all invertebrate animals (after nematodes). They include the crustaceans such as crabs, lobsters or isopods, insects such as wasps, dragonflies or ants, myriapods such as the centipedes and millipedes as well as the chelicerates, which include the arachnids - the spiders, scorpions, the horseshoe crabs, and the pycnogonids and related animal groups. An important extinct group of arthropods were the trilobites.

There are basically two differenty hypotheses about the evolutionary origin of arthropods: The articulata concept sees their closest relatives in annelid worms, citing the segmentation as the most important shared character. The ecdysozoa concept allies arthropods instead with other invertebrates that periodically shed their cuticles or exoskeletons, including the omnipresent nematodes, or "thread worms," and their relatives, the nematomorphs, also known as "horsehair worms," and the Scalidophora, a group of little known worms: priapulids, or "penis worms," kinorhynchs, or "mud dragons," and loriciferans, also known as "corset worms."

Two minor phyla that are in all probability very closely related to the arthropods are tardigrades, or "water bears," and onychophores, the "velvet worms."