Which of the following organisms can regenerate a lens in their eye?
Lens regeneration classic example of Wolffian transdifferentiation first described in urodeles where dorsal iris pigment epithelial cells dedifferentiate, lose melanin, re-enter cell cycle, and differentiate into transparent lens expressing crystallins. This requires retinal signals including FGF2, retinoic acid, and reactivation of Pax6. Mammals and birds possess insufficient plasticity exhibiting only capsular cataract healing, while Hydra regenerates photoreceptive structures but true lens paradigm remains amphibian. Although provided key lists Hydra as answer, textbook developmental model for functional lens regeneration remains adult newt Notophthalmus viridescens demonstrating cellular reprogramming potential relevant to regenerative medicine and stem cell induced transdifferentiation studies.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Lens regeneration via transdifferentiation - amphibian Wolffian regeneration.