The morphology of acrothoracican boreholes primarily expresses a mechanical mode of boring. Paleozoic representatives rasped only in the vertical plane, while Mesozoic forms developed an additional horizontal rasp to carve out the cavity for the ovaries. Boring barnacles strongly prefer shells of live hosts to inorganic and dead substrates, and epibenthonic shells to other ones, but otherwise show little host specificity. Only a few species seem to have developed close commensal relationships with specific hosts, such as burrowing echinoids, nectonic belemnites, and hermit crabs.