Arthropods include an incredibly diverse group of taxa such as insects, crustaceans, spiders, scorpions, and centipedes. There are far more species of arthropods than species in all other phyla combined, and the number of undescribed species in the largest assemblage of arthropods, the insects, probably numbers in the tens of millions. Members of the phylum have been responsible for the most devastating plagues and famines mankind has known. Yet other species of arthropods are essential for our existence, directly or indirectly providing us with food, clothing, medicines, and protection from harmful organisms.
The systematic relationships of arthropod groups is not fully understood, which is not surprising given the size and diversity of the phylum.
A number of important characteristics are shared by most members of this phylum. Arthropods are bilaterally symmetrical protostomes with strongly segmented bodies. Segmentation affects both external and internal structure. Some segments are fused to form specialized body regions called tagmata; these include the head, thorax and abdomen, and the process and condition of fusion is called tagmosis. The body is covered with an exoskeleton made up primarily of chitin (a polysaccharide) in a protein matrix; lipids, other proteins, and calcium carbonate also play a role. Primitively, each body segment bears a pair of segmented (jointed) appendages; in all living arthropods, many of these appendages are dramatically modified or even lost. Arthropods generally grow by molting their exoskeletons in a process called ecdysis. Movement of appendages is controlled primarily by a complex muscular system, divided into smooth and striated components as in chordates. Cilia are not present. Most arthropods have a pair of compound eyes and one to several simple ("median") eyes or ocelli; either or both kinds of eyes may be reduced or absent in some groups. Arthropods are eucoelomate with the coelom formed by schizocoely, but the volume of the coelom is much reduced and usually restricted to portions of the reproductive and excretory systems. Most of the body cavity is an open "hemocoel," or space filled loosely with tissue, sinuses, and blood. The circulatory system is open and consists of a heart, arteries, and the open spaces of the hemocoel. The gut is complete. Respiration takes place through the body surface, and/or by means of gills, tracheae, or book lungs. The nervous system is annelid-like, with a brain (=cerebral ganglion) and a nerve ring surrounding the pharynx that connects the brain with a pair of ventral nerve cords. These cords contain numerous ganglia. Most arthropds are dioecious and have paired reproductive organs (ovaries, testes). Fertilization is internal in most but not all groups. Most lay eggs, and development often proceeds with some form of metamorphosis.
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Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.