A Because Astrology does not Demonstrate effectiveness
The answer to what structures might be found in the cranial portion of the midsagittal plane is the umbilicus and the bladder also you can also find the feet and the ankles, an in the structure you can also find of course the nose and the mouth, even the hand and the feet are included.
Does Natural selection or evolution involve with this? it sounds like spiders and other Insects or bugs that have evolved to be able to have limbs in order to survive in a environments that would require extra limbs. probably from genes being passed on and also being mutated positively in order for the newer species to better adapt. the pair of legs could have been increased because of the environment that requires it and also as the mating changes it's basically survival of the fittest.