In members of the Chelicerata, the body consists of a cephalothorax and an abdomen. Unlike other groups of arthropods, no antennae or chewing mandibles are present. Apparently the segments that bear these structures in other arthropods have been lost during the evolution of chelicerates. Instead the first pair of appendages, which are located immediately anterior to the mouth are the chelicerae and these are used to manipulate food into the mouth and in spiders for injecting venom. The second pair of appendages are the pedipalps, which have a variety of functions. For instance, they are used for transferring sperm by male spiders and for holding prey in scorpions. These are followed by four pairs of walking legs. Compound eyes are lacking in all living groups except the ancient horseshoe crabs. Most chelicerates are predators, a few such as ticks suck vertebrate blood, and some such as mites feed on plant tissue. Predators such as spiders and scorpions produce toxic venoms that are used to paralyze prey and for defense.

Examine the body plans of horseshoe crabs, Class Merostomata (C & V fig. 7.78), and scorpions and spiders, Class Arachnida (C & V fig 7.79-7.83). Note differences in the structure of chelicerae, pedipalps, and walking legs in each group.