Chelicerates have adapted to many ways of life, ranging from marine predators to air-breathing parasites, and can look remarkably different as a result. However, they all share some common features which unite them as a group.
Since chelicerates belong to the Arthropoda, they show characteristics common to all arthropods. They show a segmented body, a chitinous exoskeleton (which they moult) and jointed legs.
Whereas many arthropods show three main body regions (head, thorax and abdomen), chelicerates have two: the cephalothorax (also called the prosoma) and the abdomen (also called the opisthosoma). This is the result of a 'fusing' of the head and thorax regions. The abdomen often bears an unsegmented telson ('tail spine').
Chelicerates are also defined by the arrangement and specialisation
of their appendages. Unusually for arthropods, they have no
antennae. Usually, six pairs of appendages occur on the six
segments of the prosoma. The first pair (at the anterior, i.e.
'head end') is distinctive, and forms the chelicerae (for
which the group is named). The chelicerae are often modified into
claw-like structures. They are followed by the pedipalps,
which are also commonly modified. Depending on the group, the
chelicerae and pedipalps may be modified for feeding, sensing,
defence, reproduction or locomotion. The remaining four pairs
of appendages on the prosoma are usually modified for walking.
Often it is difficult to identify chelicerate characters (particularly the presence or absence of chelicerae) in fossil specimens. Therefore, the following characters are also helpful as a guide: the presence of median (simple) eyes (as differentiated from compound eyes), and differentiation of the opisthosoma into a preabdomen and postabdomen (Dunlop and Selden, 1997).
as shown by the eurypterid Onychopterella augusti, showing dorsal (upper) and ventral (lower) surfaces
(image from Braddy et al. (1995); labelled using scheme of Brusca and Brusca (1990)).
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