It is estimated that the number of living arthropod species in known space is probably in the hundreds of millions. Arthropods have a stiff carapace (or exoskeleton) made of largely calcium carbonate. This extrusion is a compound known as chitin. They have segmented bodies and show various patterns of exterior fusion; these form integrated units of skeletal growth and protect the soft inner body from damage and exposure.
The phylum takes its name from the distinctive jointed appendages. These limbs are modified in a number of unique ways to form antennas, offensive appendages, mouth parts (including fangs) and reproductive organs. Arthropods first appeared in the sea on Earth, but this origin cannot be assumed for species discovered on other worlds.
Arthropods have traditionally been divided into five types – Hexapoda, Crustacea, Arachnida, Myriapoda, and Megalopoda. An Arthropod is any animal that belongs to the major division, or phylum, of the animal kingdom called Arthropoda. This term is formed from two Greek words (arthos and podae) and means jointed feet. Technically speaking the legs rather than the feet are jointed. All Arthropoda have legs; these appendages are one of their most distinctive characteristics.
An expanded list of the types of arthropods as noted above is presented here.
- Hexapoda: Insects, including cockroaches, beetles, bees, butterﬂies and related species.
- Crustacea: Most aquatic arthropods, including crabs, lobsters, shrimps and barnacles.
- Arachnida: Eight legged arthropods, including mites, ticks, spiders and scorpions.
- Myriapoda: Chillopods, including centipedes and millipedes.
- Megalopoda: The class given to the recently discovered and inappropriately named ‘Arachnids’. These extraterrestrial arthropods differ in structure enough from hexapods (their closest scientiﬁc relative) to warrant a unique classiﬁcation.