Phylum Arthropoda

Phylum Arthropoda (Greek arthros, joint + podos, foot) is by a very wide margin the largest phylum in the entire Animal Kingdom, containing more described species than every other phylum of animals combined — insects alone account for over two-thirds of all known animal species. The phylum is named for its single most defining feature: jointed appendages, articulated legs, antennae and mouthparts that allow highly precise, localised movement at each joint. The body is bilaterally symmetrical, triploblastic and coelomate, and is externally segmented, the segments being grouped, to varying degrees in different classes, into functional regions such as a head, thorax and abdomen (in insects) or a fused cephalothorax and abdomen (in many crustaceans and arachnids).

The whole body is enclosed by a hard, chitinous exoskeleton, which protects the internal organs and provides surface area for muscle attachment, but which cannot grow with the animal and must therefore be periodically shed and replaced in a process called moulting (ecdysis). The circulatory system is of the open type: blood (more precisely called haemolymph, since it is not confined to vessels) is pumped by a dorsal heart into a system of open sinuses collectively called the haemocoel, through which it bathes the tissues directly before draining back toward the heart. Respiratory organs vary strikingly with habitat: aquatic forms such as prawns respire through gills, terrestrial arachnids through book lungs, and terrestrial insects through a branching network of air tubes called tracheae that open to the exterior via spiracles — the tracheal system is so efficient that it delivers oxygen almost directly to the tissues, bypassing the blood for respiratory gas transport altogether. Most arthropods possess compound eyes, built of numerous individual visual units (ommatidia), well suited to detecting motion. …