artropoda

Arthropoda, or arthropods, is the largest phylum of animals, including insects, arachnids, crustaceans, and myriapods, as well as extinct relatives. Members are characterized by a chitinous exoskeleton, a segmented body organized into tagmata, and paired jointed appendages. They have a dorsally located heart and an open circulatory system, a ventral nerve cord, and diverse sensory organs such as compound eyes and antennae. Most arthropods grow by ecdysis, a molting process that replaces the exoskeleton as they increase in size. Respiratory adaptations are diverse, including gills in aquatic forms, book lungs in some arachnids, and a network of tracheae in insects.

Arthropoda is divided into several major subphyla. Trilobitomorpha consists of extinct forms. Chelicerata includes spiders, scorpions, mites, and horseshoe crabs; Crustacea includes crabs, lobsters, shrimp, barnacles, and relatives; Hexapoda comprises insects and their close relatives. Insects are the most diverse group, with a wide range of body plans and life cycles, including complete (holometabolous) and incomplete (hemimetabolous) metamorphosis.

Ecology and evolution: Arthropods occupy nearly every habitat and play crucial roles as pollinators, scavengers, predators, parasites, and prey for other animals. They first appear in the fossil record in the early Cambrian and show extensive adaptive radiation. Their largely modular body plan, exoskeleton, and diverse life histories have contributed to their ecological prominence and economic importance in agriculture, medicine, and ecology.