Inside arthropod bodies are incredibly powerful muscles that make mouthparts chew, antennae wiggle, legs dig, and wings fly. The nervous system helps to coordinate these and other movements. The brain is located inside the head. Trailing behind the brain is a nerve chord that runs along the entire length
of the arthropod's underside. Along the nerve chord are bundles of nerves called ganglia (GANG-lee-uh) that help control the various parts of the body. A pair of ganglia controls each pair of appendages. All abdominal segments have a pair of ganglia except in millipedes, where each segment has two pairs. Most body segments of millipedes are actually two segments joined as one. This is also why millipedes have two pairs of legs on most of their body segments.
The blood of fishes, amphibians, reptiles, birds, and mammals circulates inside arteries and veins. This is called a closed circulatory system. But arthropods have an open circulatory system. They have a tube that runs along their backs. A series of pumps, or hearts, inside the abdomen, or body trunk, pumps the blood forward in the tube. Eventually, it spills out behind the head into various body cavities. The blood usually does not carry oxygen, but it does carry nutrients and chemicals called hormones (HOR-moans) that help the body to function. All the tissues and organs are bathed in blood. The blood eventually moves back to the abdomen where it enters the tube through tiny holes located between the hearts.
Arthropods do not have lungs. The respiratory, or breathing, system of most species living on land is made up of a series of holes and tubes. Oxygen enters the body through a series of
holes along the sides of the body called spiracles (SPIH-reh-kuls). Each spiracle is attached to a network of tubes, or trachea (TRAY-key-uh). The trachea carry oxygen throughout the body. Carbon dioxide, a waste product of living tissues, is expelled out of the body through the same system. Some spiders have a tracheal system, but most use book lungs. Book lungs are made up of folded tissue inside the abdomen that resembles the pages of a book. Aquatic insects either trap a bubble of air over their spiracles or use gills or gill-like structures. A very thin layer of exoskeleton covers the gills, allowing dissolved oxygen in the water to pass through and enter the tracheal system. Some species have no respiratory system at all. Instead, oxygen in the water simply seeps in all over their bodies.
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