- Name Meaning: Jointed foot
- English Common Name: Arthropods
- Major distinguishing characteristics: Chitin exoskeleton
- Approximate number of species described: 1,134,000+
Counted by number and diversity of species, arthropods are the most successful Phylum of animals. Over half known species are arthropods.
Arthropods live in most habitats on Earth. They are present in the deepest ocean trenches and have been found high in the atmosphere.
The Phylum Arthropoda is divided into five Subphyla (four extant, one extinct). The Subphyla are divided into 16 Classes. There are an additional 6 extinct Classes that are so far unable to be placed within Subphyla.
- Subphylum Trilobitomorpha (extinct)
- Trilobita (trilobites, extinct)
- Subphylum Chelicerata
- Arachnida (spiders, scorpions, ticks, etc.)
- Merostomata (horseshoe crabs, eurypterids) - eurypterids are extinct
- Pycnogonida (sea spiders)
- Subphylum Myriapoda
- Chilopoda (centipedes)
- Diplopoda (millipedes)
- Symphyla (resemble centipedes)
- Subphylum Crustacea
- Branchiopoda (brine shrimp)
- Remipedia (blind crustaceans)
- Cephalocarida (horseshoe shrimp)
- Maxillopoda (barnacles, copepods, fish lice)
- Ostracoda (seed shrimp)
- Malacostraca (lobsters, crabs, shrimp)
- Subphylum Hexapoda
- Entognatha (springtails)
- Insecta (insects)
It's uncertain how these extinct groups fit in with the rest of the Arthropoda, but they are arthropods.
Being a large and diverse group, the anatomy of arthropods is also somewhat diverse.
Arthropods have an exoskeleton primarily made of chiton, a fibrous material which is strong, yet somewhat flexible. Their bodies are segmented, with some specialization of segments.
Though they have hearts to move fluids in the body around, Arthropods have open circulatory systems. Their blood flows freely among their tissues, though some have some open ended arteries.
The Fossil Record
Arthropods appear in the fossil record as early as the Vendian, 543 million years ago. Their diversification was rapid during the Cambrian, and there are several represented in the Burgess Shale.
There are several extinct groups, the most well known being the trilobites which evolved by about 521 million years ago and became extinct about 252 million years ago at the end of the Permian.