Symmetry in which any diameter bisects the object into two essentially identical halves. In bilateral symmetry, there is only one plane about which the structure has two mirror-image halves.

[Cnidaria] In many hydromedusae, despite the cross-shaped arrangement of the radial canals, the bell can be so divided along any diameter: the division does not have to be along a radial canal. Such hydromedusae are also said to have tetraradiate symmetry.