Classified under the phylum Arthopoda.

Body composed of two parts (or tagmata): Prosoma and Opisthosoma.

Prosoma comprises of precheliceral metameres and the segments I-VI
Opisthosoma comprises ancestrally 10-13 segments and, in several cases, the telson

The CNS lacks a deutocerebrum, and there are no antennae and no mandibles

Ancestrally, there are six pairs of prosomatic appendages:
1. a pair of chelicerae
2. a pair of palpi
3-6. legs

Infraphylum Cheliceriformes
Superclass Sidneyiida (extinct)
Superclass Emeraldellida (extinct)
Superclass Sanctacarida (extinct)
Superclass Chelicerata