Fossil records suggest that arachnids were among the first animals to live on land, switching from water- to air-breathing. The oldest known arachnid fossils date from the Silurian Period, more than 417 million years ago. It is during this period of time that scorpions (order Scorpionida) appear to have left the water for life on land. Many paleontology experts presume that scorpions were the first animals to make the transition from water to land. In fact, the histological resemblance between the gills of king crabs and the lungs of scorpions help to support this hypothesis. However, the sub-phylum Cheliceraformes, as a whole, spent many millions of years in the water before it became terrestrial. More than 60,000 species of arachnids are described, although many species, especially mites, remain undiscovered or discovered-but-not-yet-described. Spiders, mites, and ticks constitute the largest and most diverse orders of arachnids. Among the extant species, scorpions are known to have had a long maritime history that continued well after some of them switched to living on land. The marine-living scorpions, at that time, were very large, some up to 3.3 ft (1 m) in length. The harvestmen (daddy longlegs) are also believed to have had a pre-terres-trial history in the sea.
Currently, arachnids constitute the subclass Arachnida, in the phylum Arthropoda. The subclass is divided into 11 distinct orders: Acari (mites, chiggers, and ticks), Amblypygi (tailless whip scorpions), Araneae (spiders), Opiliones (daddy longlegs), Palpigradi (palpigrades), Pseudoscorpiones (false scorpions), Ricinulei (ricinuleids), Schizomida (micro whip scorpions or schizomids), Scorpionida (scorpions), Solpugida (wind scorpions or solifugids), and Uropygi (whip scorpions and vinegaroons). Many scientists now categorize Arachnida at the class level.
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