The coiled shells of goniatites, though hard, were not unchanging. They sporadically grew to accommodate the soft body of the animal. The thin walls between the internal chambers of the shell are known as septa. As goniatites increased in size, they would move their bodies forward in the shell. At the same time, they secreted new septa behind them, thereby adding another chamber to their shells. Where each septum contacted the wall of the outer shell, sutures are visible as a series of narrow lines on the surface of the shell fossils. The sutures of goniatites have a characteristically zigzag shape.