When a hydra is fragmented into various pieces, each piece is capable of regenerating into a complete, smaller hydra. This regenerative mode involves existing cells re-patterning themselves without the extensive formation of new cells, reconstructing the missing structures and restoring functional morphology.

What Is Morphallaxis?

• Morphallaxis is regeneration primarily through tissue remodeling and reorganization of existing cells, not dependent on a blastema or significant cell division.

• During hydra regeneration, each fragment adjusts its structure and polarity, forming all necessary parts such as head and foot through positional cues and cellular repatterning.

• The regrown hydra is generally smaller compared to the original but maintains all functional parts.

Why Hydra Regeneration Is Morphallaxis

• The regeneration is achieved by transdifferentiation (cells changing their type) and morphogenetic gradients controlling the formation of new structures.

• There is minimal cell proliferation in the initial phases of regeneration; the process relies on remodeling existing tissues.

• This type of regeneration contrasts with epimorphosis seen in organisms like salamanders, where regeneration involves significant cell proliferation and blastema formation.

Explanation of the Options

• (1) Regeneration: This is a broad term and can include multiple modes, but does not specify the mechanism.

• (2) Morphallaxis: Correct answer as it describes regrowth by repatterning without new tissue growth, typical of hydra.

• (3) Epimorphogenesis: Incorrect for hydra; epimorphosis involves blastema formation and cell division, which hydra do not primarily use.

• (4) Teratogenesis: Refers to abnormal development or birth defects, unrelated to normal regenerative processes.

Summary

The mode of regeneration in hydra, involving fragments repatterning themselves into complete organisms without large-scale proliferation, is termed morphallaxis. This process exemplifies regeneration via tissue remodeling and cellular transformation, making hydra a vital model for this form of regeneration.

Final Answer:
(2) Morphallaxis