Regeneration - Morphallaxis and Epimorphosis

Regeneration is the regrowth of an injured or missing region of the body, and it was first studied scientifically in Hydra by Abraham Trembley as far back as 1740. Two distinct types of regeneration are recognised. In morphallaxis, an entire new body can grow from just a small fragment of the original animal, as seen in Hydra and Planaria - if a Hydra is accidentally cut into several pieces, each individual piece is capable of regenerating all its missing parts and developing into a whole, smaller but complete new Hydra, with each piece retaining its original polarity as it does so (the end that was originally oral develops a new mouth and tentacles, while the end that was originally aboral develops a new basal disc).

Epimorphosis, by contrast, is the replacement of specific lost body parts rather than the regrowth of an entire body, and it comes in two forms: reparative regeneration, in which only certain damaged tissue is regrown (rather than an entire structure), and restorative regeneration, in which whole severed body parts can be regrown - illustrated by the regeneration of a starfish's arm and by the regrowth of a wall lizard's tail after autotomy. …