Phylum Arthropoda — Arthropods
Arthropods are bilaterally symmetrical invertebrates with segmented bodies, jointed appendages, and a rigid external skeleton made primarily of chitin, often strengthened with calcium salts. This exoskeleton must be shed periodically through moulting to allow growth. They are the most diverse and abundant animals on Earth, representing over 80% of all known species and occupying almost every habitat, from deep oceans to high mountains and urban environments.
The name Arthropoda derives from the Greek arthron (joint) and pous (foot), referring to their distinct jointed limbs. Arthropods range from microscopic forms such as Stygotantulus stocki (94 μm) to giants like the Japanese spider crab, Macrocheira kaempferi, with a leg span of up to 3.8 m. Their success is linked to their adaptable body plan, highly developed sensory systems, and remarkable ecological versatility.
Subdivision of the Phylum
Arthropods are traditionally divided into four major subphyla based on differences in body structure, appendages, and evolutionary relationships.
Subphylum Chelicerata — Chelicerates
Chelicerates are arthropods that lack antennae and possess specialised mouthparts known as chelicerae. Most members are terrestrial predators, though some marine forms also exist.
- Class Arachnida — Arachnids (spiders, scorpions, ticks, mites, and their relatives), primarily terrestrial arthropods with eight legs.
- Class Merostomata — Horseshoe Crabs, marine chelicerates that retain many primitive features.
- Class Pycnogonida — Pycnogonids (sea spiders), unusual marine arthropods with very long legs and reduced bodies.
Subphylum Crustacea — Crustaceans
Crustaceans are mostly aquatic arthropods characterised by two pairs of antennae and a wide variety of body forms. They include many familiar marine and freshwater animals such as crabs, lobsters, shrimp, barnacles, and water fleas.
Superclass Multicrustacea
A major group of crustaceans that includes many familiar marine species.
- Class Hexanauplia — A group that includes copepods and barnacles, many of which are extremely abundant in marine plankton or live attached to hard surfaces.
- Class Malacostraca — The largest class of crustaceans, containing crabs, lobsters, shrimp, krill, mantis shrimp, and woodlice.
Superclass Oligostraca
A lineage containing several smaller crustacean groups, including both free-living and parasitic species.
- Class Ichthyostraca — Mostly parasitic crustaceans that live on fish and other marine animals.
- Class Ostracoda — Small crustaceans enclosed within a clam-like shell, often called seed shrimp.
Other Classes
These crustacean classes are not placed within the two superclasses above.
- Class Branchiopoda — Mostly freshwater crustaceans including fairy shrimp and water fleas.
- Class Cephalocarida — Tiny marine crustaceans sometimes known as horseshoe shrimp.
- Class Remipedia — Rare cave-dwelling crustaceans found in submerged coastal cave systems.
Subphylum Hexapoda — Hexapods
Hexapods are terrestrial arthropods characterised by having six legs and a body divided into head, thorax, and abdomen. This group contains the most diverse class of animals on Earth.
- Class Insecta – The insects; an enormously diverse group including beetles, flies, butterflies, bees, and many other familiar species.
- Class Entognatha – Small soil-dwelling hexapods such as springtails and their relatives, with mouthparts enclosed within the head.
Subphylum Myriapoda — Myriapods
Myriapods are terrestrial arthropods with elongated bodies composed of many segments, typically bearing numerous legs.
- Class Chilopoda – Centipedes; fast-moving predators with one pair of legs per body segment.
- Class Diplopoda – Millipedes; mostly slow-moving detritivores with two pairs of legs per segment.
- Class Pauropoda – Small, soft-bodied soil-dwelling myriapods rarely encountered without magnification.
- Class Symphyla – Tiny soil arthropods resembling small centipedes, commonly known as garden centipedes.
