The arthropod group Arachnida includes the spiders, scorpions, ticks, and their kin. Their heads and thorax are fused and protected by a carapace, or hard outer covering. Arachnids have from zero to four pairs of eyes, eight legs, and a complex set of anterior appendages associated with grabbing food, tearing it apart, and preparing it for ingestion.
The appendages used by arachnids for securing food are notoriously effective and deadly. A pair of smaller appendages, called che-licerae, is closest to the mouth. The chelicerae are used to tear food apart. A set of longer appendages, called the pedipalps, is also fixed to the front of the arachnid; these appendages are used to grab and hold food so that it can be torn apart. At the base of these appendages are mandibles, which the animal uses to reduce food to smaller pieces before it is ingested. In scorpions, these appendages generally are covered with a hard protective cover; the pedipalps, which are oversized, terminate with a grasping claw. In spiders, the pedipalps are generally small so as not to interfere with the legs. The pedipalps of spiders sometimes include grappling hooks for securing prey, and the chelicerae of spiders are fangs.
Arachnids have an intriguing evolutionary history. They undoubtedly began as marine creatures that adapted to land. Most living arachnids are terrestrial species. Those living arachnids that live in the water are derived from terrestrial ancestors, so it is clear that some branches of the arachnid family tree returned to the water after having already adapted to life on land.
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