Subphylum Chelicerata

Few invertebrates on land or sea are so often miscast in a sinister role as the spider, which along with mites, sea spiders, and horseshoe crabs comprises the 77,000 described species in the subphylum Chelicerata. Indeed, most arachnids and all other chelicerates either are harmless to humans or are actually quite helpful in their roles as predators and parasites of insects or as decomposers of terrestrial litter. Although the subphylum evolved in the sea, the ocean now supports only the five species of horseshoe crabs (class Merostomata), a thousand or so species of sea spiders (class Pycnogonida), and a few mites.

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