![]() The harvestmen ( daddy longlegs) are also believed to have had a pre-terrestrial history in the sea.Ĭurrently, arachnids constitute the subclass Arachnida, in the phylum Arthropoda. ![]() The marine-living scorpions, at that time, were very large, some up to 3.3 ft (1 m) in length. 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. Spiders, mites, and ticks constitute the largest and most diverse orders of arachnids. More than 60,000 species of arachnids are described, although many species, especially mites, remain undiscovered or discoveredbut-not-yet-described. However, the subphylum Cheliceraformes, as a whole, spent many millions of years in the water before it became terrestrial. In fact, the histological resemblance between the gills of king crabs and the lungs of scorpions help to support this hypothesis. Many paleontology experts presume that scorpions were the first animals to make the transition from water to land. It is during this period of time that scorpions (order Scorpionida) appear to have left the water for life on land. The oldest known arachnid fossils date from the Silurian Period, more than 417 million years ago. Thumbnail description Highly recognizable and populous eight-legged invertebrates with two body parts (a prosoma and an abdomen), pedipalps, book lungs or tracheae, sometimes poisonous fangs, and generally the ability to produce silk they are terrestrial chelicerates (invertebrates with pincershaped mouthparts) Evolution and systematicsįossil records suggest that arachnids were among the first animals to live on land, switching from water- to air-breathing.
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