While Butterfield agreed that the dark patches round the foot served as gills, he denied that they were similar in structure and mode of development to molluscan ctenidia. In his opinion the flattened remains of "Odontogriphus" were formed by relatively tough extracellular secretions, such as jaws, bristles and toughened skin, and do not include purely or primarily cellular tissues, such as muscles or gonads. He therefore thought the respiratory organs round the edge of "Odontogriphus"’ foot could not be molluscan ctenidia, since these are covered by purely cellular tissue. Instead he suggested that they might be brachiopod lophophores, which are feeding organs that contain a lot of extracellular material, or polychaete branchiae, which are respiratory organs composed largely of non-cellular cuticle – both of these types of structure have been found in the Burgess Shale, in which all the known specimens of "Odontogriphus" have been discovered.