arthropoda

Arthropoda is the largest phylum in the animal kingdom, with more described species than any other group. Arthropods have segmented bodies, an exoskeleton made of chitin and proteins, and jointed appendages. They possess a ventral nerve cord and a dorsal heart, and their circulatory system is open. Respiration varies: some use gills, others book lungs or a network of tracheae. Growth is by ecdysis, or molting, regulated by hormonal signals in most groups.

The phylum is divided into four major subphyla: Chelicerata (spiders, scorpions, horseshoe crabs), Myriapoda (centipedes and millipedes), Crustacea (crabs, lobsters, shrimp, barnacles), and Hexapoda (insects and their relatives). Insects, within Hexapoda, are the most diverse group and often possess wings and various forms of metamorphosis. Arthropods show tremendous diversity in body plans, including cephalothorax in some chelicerates and a wide range of sensory organs.

They occupy nearly every habitat and play crucial ecological roles as pollinators, predators, herbivores, detritivores, and parasites. Reproduction is diverse: most species are dioecious, with internal fertilization common in many groups; development includes direct and indirect life cycles, with incomplete (hemimetabolous) or complete (holometabolous) metamorphosis. Excretion is mediated by Malpighian tubules in terrestrial forms and by analogous structures in crustaceans; respiration varies with modality.

Human relevance and fossil record: Arthropods have a long fossil record dating to the Cambrian and are central to modern ecosystems and economies. They are pests, pollinators, and a major food source for humans and other animals; they also serve as model organisms in biology and medicine. The phylum's success is attributed to modular body plans, an exoskeleton, versatile jointed appendages, and, in insects, wings.