The morphology of the setae, on the mouthparts and the cirri of 33 barnacle species, was investigated by scanning electron microscopy. The species examined were selected from a range of pedunculate and balanomorph families living in a variety of habitats from the littoral zone to deep-sea vents. It was found that the setae could be divided into five groups: simple, denticulate, setulate, plumo-denticulate and aesthetascs. The categories were subdivided resulting in 43 setal types being recognized. Balanomorph cirripedes were found to have a greater variety of setae than pedunculates. Setulate setae on the cirri, and specialized setae on the mandibular palps and the first maxillae tended to be associated with the development of the balanomorph form. Setal type was correlated with taxonomy, whether based on shell structure and soft body morphology or DNA analysis, but with some anomalies which deserve further investigation. The problems of reconciling the proposed cirripede setal categories with those described in the Decapoda are discussed.
