The bilateral symmetry of bilaterians is a complex trait which develops due to the expression of many genes. The bilateria have two axes of polarity. The first is an anterior - posterior (AP) axis which can be visualised as an imaginary axis running from the head or mouth to the tail or other end of an organism. The second is the dorsal - ventral (DV) axis which runs perpendicular to the AP axis. During development the AP axis is always specified before the DV axis, which is known as the second embryonic axis.