Surely one of the most successful groups of animals in the history of this planet. There are arachnids on every continent (except Antarctica, obviously). Arachnids include spiders, scorpions, mites, ticks and the like. Though it is very rare for soft-bodied animals to survive, in fossil form, the oldest known fossil spider is 380 million years old, and there are even older fossil scorpions and sea scorpions known. Whenever I watch a tarantula or scorpion I can't help feeling I'm being given a viewpoint on an early age of life on this planet.
An example of an arachnid: a tarantula, a member of the oldest group of spiders which did not spin webs, merely used silk to line their burrows.