approximately perpendicular to orthogonal to anatomical axis anterior-posterior axis A-P axis AP axis An axis that extends through an organism from head end to opposite end of body or tail. BSPO:0000013 In sponges, AP is used to indicate the direction of movement [in larval stage] (as it is in other metazoans that move, e.g., the basal bilaterians). [PM] anteroposterior axis cephalocaudal axis craniocaudal axis longitudinal axis rostral/caudal rostrocaudal axis spatial dorsal-ventral axis transverse plane