A new homologous series of achiral five-ring bent-core mesogens is presented. The mesophase behaviour has been studied by polarizing microscopy, differential scanning calorimetry, X-ray diffraction and electro-optical measurements. The homologues with shorter terminal chains (C₈, C₁₂) form an SmCP_A phase, homologues with longer chains (C₁₄, C₁₆, C₁₈) show liquid-crystalline tetramorphism with a sequence SmA–SmC_SP_A–Col_ob–SmC_SP_A. The SmCP_A phases are structurally identical and differ only in the mechanism of polar switching. The switching of the high-temperature SmC_SP_A phase takes place through the collective rotation of the molecules around their long axes whereas in the reentrant SmC_SP_A phase the switching is based on the director rotation around the tilt cone. By the application of an electric field the Col_ob phase can be irreversibly transformed to an SmCP_A phase. In a limited temperature range the SmA phase shows a reversible field-induced transition into the SmC_SP_F phase. The electro-optical response of the mesophases is discussed on the basis of the structural features of these phases.