arachnid ərăk´nĭd [key], mainly terrestrial arthropod of the class Arachnida, including the spider, scorpion, mite, tick, harvestman (daddy longlegs), and a few minor groups. The body is divided into a cephalothorax with six pairs of appendages, and an abdomen. The first two pairs of appendages are used to kill and crush prey (most arachnids being carnivorous); the remaining four pairs are walking legs. Arachnids have simple eyes and no antennae but are equipped with sensory bristles. Some respire with air tubes, but most possess primitive respiratory organs called book lungs. Arachnids are classified in the phylum Arthropoda, subphylum Chelicerata, class Arachnida.
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