SUMMARYArthropods (Arthropoda) are a highly-successful group of invertebrate animals that includes centipedes, millipedes, spiders, mites, horseshoe crabs, scorpions, insects and crustaceans. In terms of species diversity, arthropods are second to none. The muscle structure of arthropods is more complex than that of most other invertebrates. Arthropods have longitudinal and circular bands of muscle tissue as well as isolated muscles that together enable a wide range of movements. Additionally, arthropods have a well-developed nervous system. In more advanced arthropods, the nervous system consists of a brain and a double nerve cord. More primitive arthropods do not have a brain but instead have ganglia either located in each segment of their body or in a ganglionic mass near the head.
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HISTORY OF ARTHROPODSThe first arthropods appeared during the early Cambrian Period. They are thought to have evolved from annelids between 500 and 600 million years ago. Perhaps the best known early arthropods were the Triolobites, creatures that lived on the muddy bottoms of shallow marine environments. Trilobites disappeared about 225 million years ago during the Permian Period. The reason for their extinction is not known. The first land animal was a myriapod arthropod. Based on fossil evidence, these arthropods took to a life on land between 490 and 443 million years ago.
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