Arthropod

Arthropods are a diverse group of invertebrate animals belonging to the phylum Arthropoda. They are distinguished by a hard external skeleton (exoskeleton) made of chitin, a segmented body, and jointed appendages. With more described species than any other animal group, arthropods inhabit nearly every environment, from deep oceans to high mountains.

The typical arthropod body is organized into tagmata, often including a head, thorax, and abdomen, and it grows by molting as it sheds its exoskeleton. They exhibit an open circulatory system, a ventral nerve cord, and a dorsal brain. Most possess compound eyes and a variety of sensory organs.

Respiration varies by lineage: aquatic forms may use gills or book lungs; terrestrial insects typically breathe through a network of tracheae. Excretion varies, with malpighian tubules in insects and maxillary glands in some crustaceans. Reproduction is predominantly sexual, with many species exhibiting metamorphosis that changes body form during development.

Arthropods occupy diverse ecological roles. Insects pollinate crops and wild plants, crustaceans and some arachnids occupy aquatic food webs, and many arthropods decompose organic matter. They are also important prey and, in some cases, vectors of disease or agricultural pests. Humans study arthropods as model organisms and for their economic importance.

Arthropods include four major subphyla: Chelicerata (spiders, scorpions, horseshoe crabs), Crustacea (crabs, lobsters, shrimp, copepods), Hexapoda (insects and their relatives), and Myriapoda (centipedes and millipedes). The group originated in the early to mid-Cambrian period. The exoskeleton and jointed limbs facilitated wide diversification.