homologous_to cuttlebone Predominately calcareous internal shell of sepiids (cuttlefish). It is composed of many chambers and functions primarily as a buoyancy organ. The cuttlebone is homologous with the phragmocone of <strong>Spirula</strong>, <strong>Nautilus</strong> and many fossil cephalopods. sepion A calcareous oblong supporting part of the mantle of cuttlefishes phragmacone organism subdivision