Answer:
The 'Largest Arthropod ever Known to Exist' was either the Eurypterid (sea scorpion) 'Jaekelopterus' or the 'Carboniferous millipede Arthropleura', which were both between 2.5 and 2.6 m (8.2-8.5 ft) in length. They were closely followed by 'Pterygotus', an aquatic Eurypterid that was up to 2.3 m (7.5 ft) in length. The closest living relatives of Eurypterida are the Arachnids, Horseshoe Crabs, and Sea Spiders.
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