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and covered in hairs and spines. These are thought to be vestiges of the limbs of an absorbed first opisthosomal segment.

The opisthosoma is divided into a forward mesosoma, with flattened appendages, and a metasoma at the rear, which has no appendages. In modern forms, the whole of the opisthosoma is fused into a single unsegmented structure. The underside of the opisthosoma carries the genital openings and five pairs of flap-like gills.

The opisthosoma terminates in a long caudal spine, commonly referred to as a telson

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