that in most sub-groups perform sensory functions, while the remaining four cephalothorax segments (somite 4 to 6) have pairs of legs. In basal forms the ocular somite has a pair of compound eyes on the sides and four pigment-cup ocelli ("little eyes") in the middle. The mouth is between somite 1 and 2 (chelicerae and pedipalps).

The opisthosoma consists of thirteen or fewer segments, may or