Arachnids are a class (Arachnida) of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although the front pair of legs in some species has converted to a sensory function, while in other species, different appendages can grow large enough to take on the appearance of extra pairs of legs. The term is derived from the Greek word ἀράχνη (aráchnē), meaning "spider".
Almost all extant arachnids are terrestrial. However, some inhabit freshwater environments and, with the exception of the pelagic zone, marine environments as well. They comprise over 100,000 named species, including spiders, scorpions, harvestmen, ticks, mites and Solifugae.
In Walking with... SeriesEditMegalograptus, a medium-sized sea scorpion. It was an intermediate predator, hunting trilobites, but prey itself for Cameroceras and similar giant mollusks - which was why it began to come on land to lay eggs and to escape predation. Brontoscorpio, a distant relative to the modern scorpions, and Pterygotus, a giant sea scorpion. These animals were shown to be the dominant predators of the Silurian period, preying on Cephalaspis and similar creatures, taking over the role of the giant mollusks. This episode also shows a Devonian terrestrial scorpion (played by a modern scorpion) that was, instead, prey for the first terrestrial vertebrates, such as Hynerpeton.
The first half of this episode featured Mesothelae, a giant spider. It was shown preying on small reptiles like Petrolacosaurus, but it was too slow to outrun forest fires of the Carboniferous and too big to survive when the oxygen-rich world of the Carboniferous came to an end.
The book version of this episode features a whip scorpion that got eaten by a Coelophysis.