Class of terrestrial chelicerates (Chelicerata) which have book lungs or tracheae derived from gills, indicating their aquatic derivation. Of recent terrestrial animals, the arachnids are probably the oldest known class, scorpions having been recorded from the Silurian period. A Silurian scorpion, Palaeophonus nuncius, was perhaps the first terrestrial animal. The first fossil spiders are known from the Devonian. The class is extremely diverse, but except in the mites the body is in two portions: the prosoma (anterior portion) which bears the four pairs of legs, the eyes, the pedipalps (second pair of appendages), and the chelicerae (first pair of appendages, usually pincer-like); and the opisthosoma (posterior portion) which contains most of the internal organs and glands. The two portions may be broadly jointed, or connected by a stalk or pedicel. The prosoma has a dorsal shield or carapace, and the opisthosoma is segmented in most orders, but not in spiders and mites and only very weakly in harvestmen. The number of eyes varies, and can be as many as twelve in some scorpions.