RARE REVERSIBILITY. “Also, the probability that this sequence will occasionally reverse will probably vanish soon, as demonstrated by the life cycles of most animal species. Exceptions to this rule are indeed extremely rare. Best known is the case of the hydrozoan Turritopsis nutricula, where a medusa, under certain conditions, is able to revert to a polyp.”
http://rstb.royalsocietypublishing.org/content/365/1540/631.short