CLASSES OF ARTHROPODS

Class Chelicerata
Subclass Merostomata (horseshoe crabs)
Subclass Arachnida (spiders, scorpions, mites, ticks) Horseshoe crab class Arachnida , subclass Merostomata , from Pratt, 1923.
The Chelicerates are an ancient group, originating from at least early Cambrian times, and are well represented in the fossil record. They include the extinct Eurypterids , or water scorpions , which reached sizes of more than 2 meters in length. The most familiar living chelicerates are the spiders, scorpions , ticks, and mites (subclass Arachnida ). The only primarily marine group is the subclass Merostomata (horseshoe crabs).