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( Arthropoda)
Arthropods are animals belonging to the Phylum Arthropoda (from Greek ?????? arthron, "joint", and p?d?? podos"foot", which together mean "jointed feet") and include the insects, arachnids, crustaceans and allies. Arthropods are characterized by the possession of a segmented body with appendages on at least one segment. They have a dorsal heart and a ventral nervous system. All arthropods are covered by a hard exoskeleton made of chitin, a polysaccharide, which provides physical protection and resistance to desiccation. Arthropods grow by shedding this covering in what are termed molts. They are the largest phylum in the Animal Kingdom with more than a million described species making up more than 80% of all described living species,[1] and a fossil record reaching back to the late proterozoic era. Arthropods are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms. They range in size from microscopic plankton (~¼&_160;mm) up to forms several metres long. The largest living arthropod is the Japanese spider crab, with a leg span up to 3½&_160;m (12&_160;ft), and some prehistoric arthropods were even larger, such as Jaekelopterus and Arthropleura. The success of arthropods is related to their hard exoskeleton, segmentation, and jointed appendages. The appendages are used for feeding, sensory reception, defense, and locomotion. The muscle system is more or less assisted by hydraulics originated from the blood pressure, created by the hearts of the animals.[2] The hydraulic system in spiders is especially well developed. Arthropods have an open circulatory system. Haemolymph containing haemocyanin, a copper-based oxygen-carrying protein (the copper makes the blood blue, unlike humans, which use hemoglobin, which uses iron, which makes it red). The blood is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods are protostomes. There is a coelom, but it is reduced to a tiny cavity around the reproductive and excretory organs, and the dominant body cavity is a haemocoel, filled with haemolymph, which bathes the organs directly. The arthropod body is divided into a series of distinct segments, plus a pre-segmental acron, which usually supports compound and simple eyes and a post-segmental telson. These are grouped into distinct, specialised body regions called tagmata. Each segment, at least primitively, supports a pair of appendages.
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