The third appendage is modified in some pycnogonids and xiphosurids for use in reproduction, as either an ancillary pair of claspers or ovigers (Fig. 26a, b ), and this appendage is differentiated into a tactile sensory limb (Fig. 26f ) in some arachnids, including thelyphonids, while some opilionids have a differentiated fourth appendage.
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