The influence of the magnetic field on living organisms, which is strong in some species. Certain Spirillum bacteria orient themselves along the geomagnetic lines of force. Higher organisms, e.g. snails, bees, birds, porpoises, and possibly humans, appear able to sense and utilize geomagnetic field directions for purposes of orientation. Strong magnetic fields may be detrimental to health in humans, but the effects of medium strength fields have not been studied. Magnetic fields are also used for scanning biological organisms.