So... crazy creature here. Not only does this slug look like a leaf, it is able to use sunlight to produce sugars. Turns out these animals are able to uptake the chloroplasts from ingested plant matter (in this case algea) and incoperate them fuctionally and spatially into their cells!
Because the chloroplast is outside of the environment in which it evolved, it is unable to do its job without some assisting genes from its host cell , so it even transfers part of the algea's genome into its own!!! Clearly relevant to gene therapy.
Also, this animal turning into a leaf in shape and color (color reasonably being assumed to be caused by this chloroplast uptake) is a cool demonstration of structure being essential to function.