When any part of a hydra is lost, the remaining portion reorganizes and repatterns itself to regenerate a complete hydra. This type of regeneration is distinct in that it relies primarily on the reorganization of pre-existing cells and tissues rather than extensive cellular proliferation.

What Is Morphallaxis?

• Definition: Morphallaxis is a form of regeneration characterized by repatterning and remodeling of existing tissues without significant new cell growth or blastema formation.

• Cellular Activity: Instead of proliferating new cells, the hydra remodels and re-differentiates its remaining cells to restore lost structures, resulting in a smaller but fully functional organism.

• Classic Example: Hydra is one of the classic examples where morphallactic regeneration is observed. Upon cutting, each fragment can form a completely functional hydra through remodeling existing tissues.

How Morphallaxis Works in Hydra

• When hydra is cut, existing cells rearrange and change shape to close the wound and restore missing parts like the head or basal disc.

• Gradients of morphogenetic signals in the hydra body control where the head and foot regenerate, maintaining polarity.

• New tissue growth is minimal, differentiating morphallaxis from epimorphosis (which involves blastema formation and cellular proliferation).

Morphallaxis vs. Other Regeneration Types

• Epimorphosis involves cell proliferation and blastema formation (seen in salamander limb regeneration).

• Morphallaxis does not rely on new cell growth but on the transformation and repositioning of existing cells, as in hydra regeneration.

Thus, hydra regeneration is best described as morphallaxis.

Summary

The regeneration process in hydra, where a lost part is restored by the remaining tissue re-patterning itself into a complete organism, is termed morphallaxis. This process relies on the remodeling of existing cells rather than on new cell proliferation, making hydra a prime example of this regeneration mode.

Final Answer:
(2) Morphallaxis