In eurypterids, the respiratory organs were located on the ventral body wall (the underside of the opisthosoma). "Blatfüsse", evolved from opisthosomal appendages, covered the underside and created a gill chamber where the "kiemenplatten" (gill tracts) were located. Depending on the species, the eurypterid gill tract was either triangular or oval in shape and was possibly raised into a cushion-like state. The surface of this gill tract bore several spinules (small spines), which resulted in an enlarged surface area. It was composed of spongy tissue due to many invaginations in the structure.