The underside of the opisthosoma was covered in structures evolved from modified opisthosomal appendages. Throughout the opisthosoma, these structures formed plate-like structures termed "blatfüsse" (German for "leaf-feet"). These created a branchial chamber (gill tract) between preceding "blatfüsse" and the ventral surface of the opisthosoma itself, which contained the respiratory organs. The second to sixth opisthosomal segments also contained oval or triangular organs that have been interpreted as organs that aid in respiration. These organs, termed "kiemenplatten", or "gill tracts", would potentially have aided eurypterids to breath air above water, while "blatfüssen", similar to organs in modern horseshoe crabs, would cover the parts that serve for underwater respiration.