Arthropod

Arthropods make up the phylum Arthropoda, the most species-rich group of animals. More than a million species have been described, and the true number is probably several times that. The phylum includes insects; arachnids such as spiders, scorpions, ticks and mites; crustaceans such as crabs, shrimp, barnacles and woodlice; and myriapods, the centipedes and millipedes. Trilobites, once a large group of marine arthropods, died out when the Permian period closed, about 250 million years ago.

Three features unite them. The body is segmented, and groups of segments are fused into working regions such as a head, thorax and abdomen. The body is covered by a cuticle, an exoskeleton of chitin and protein, which crustaceans harden further with calcium carbonate. And the legs and other appendages are jointed, shaped into antennae, jaws, walking legs or swimming paddles. Because the exoskeleton cannot stretch, an arthropod grows by molting: it sheds the old cuticle and swells before the new one hardens. Blood flows through an open circulation, bathing the organs directly in a body cavity called the hemocoel.

Arthropods matter to people as pollinators, food and pests, and in medicine as vectors: mosquitoes, ticks and fleas carry the agents of malaria, Lyme disease and plague.