Unlike the equator and the Tropic of Cancer and the Tropic of Capricorn, the Prime Meridian and all lines of longitude are completely imaginary lines and have no significance with regard to the Earth or to its relationship with the sun.
Black hairy tongue typically results when projections on the tongue called papillae grow longer and don't shed like normal. This makes the tongue look hairy. Debris, bacteria or other organisms can collect on the papillae and result in discoloration. The cause of black hairy tongue can't always be determined.
A black hairy tongue is caused by too much bacteria or yeast growth in the mouth. The bacteria build up on tiny rounded projections called papillae. These lie along the surface of the tongue. Instead of shedding as they normally do, the papillae start to grow and lengthen, creating hair-like projections.