The chelicerates are likely the most primitive of the extant arthropod groups, and they are the simplest anatomically. Chelicerates are one of the first groups of animals known to have made the move from water to land, but their dominion over terrestrial ecosystems has not lasted. Nowadays, the spaces that once belonged to this ancient lineage mostly belong to crustaceans in water and insects (so, different crustaceans) on land. Still, chelicerates remain a major ecological force, thanks to the multitudinous mites and ticks and the prevalence of spiders and scorpions as insect predators.
And, they are magnificently weird.