Chelicerates are characterized by the absence of antennae and jaws and the presence of feeding structures (chelicera), which are modified pincerlike appendages used mainly for grasping and fragmenting food. They include spiders, scorpions, and other arachnids (class Arachnida), horseshoe crabs (class Xiphosura), and the sea spiders (class Pycnogonida) as well as the extinct giant sea scorpions (class Eurypterida). The arachnids are largely terrestrial, and the other classes marine.
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