ABSTRACT: Particle-processing, and specifically qualitative particle selection, was studied in Pecten maximus using intact and empty versions of the naturally occurring diatom Coscinodiscus perforatus. Endoscope-directed sampling allowedcomparison of particle proportions or numbers in the experimental medium, dorsal and ventral gill tracts, and labial palp pseudofeces. The general processing of particles was closely similar to that previously described for the more primitive scallopPlacopecten magellanicus, despite the large phylogenetic distance between these 2 species. The gill and the labial palps of Pecten maximus each effected particle selection (rejection of empty, cleaned frustules), both in the presence ofintact C. perforatus particles, and when only empty, cleaned frustules were presented. Particle selection operated even when small proportions (approx. 10%) of empty, cleaned C. perforatus frustules were present in the particle mixture. Thisrepresents the first direct experimental demonstration of the sites of qualitative particle selection in a heterorhabdic filibranch bivalve, and shows the process to be located in both the gills and the labial palps. The qualitative selection capacity ofthe heterorhabdic gill could be the result of either early evolution in turbid environments, or an adaptation which avoids overloading qualitative selection of the palps in this gill type.