| I don't think the reason is known but I always assumed it was the pearl and epithelium sack moving within the mullusk and sort of stretching one section of the sack as it moves.
Think of a marble inside a glob of Jelly, if you pushed that marble a little bit across a table, you'd get a "tail" section.
At least thats always how I picture it happening in my mind, someone may have some actual facts on how the form. |