Polarity in Regeneration
A distinct polarity or gradient exists in planarians (and in other animals). The meaning of this is that in any piece or segment of the body; its anterior end (head end) represents one pole and the posterior end (tail end) represents another pole. Regeneration is generally correlated with this polarity. In any excised piece the cut surface towards the head end generates a new head and the cut surface towards the hind end produces a tail. In the remaining non-chordate phyla (that is Aschelminthes, Arthropoda and Molluscs) regeneration of missing parts is minimal, and asexual reproduction including regeneration is almost non-existent.