star trek - Do Shapeshifters, like Odo, shift just shape, or composition as well?

When Odo is shaped as a humanoid, for example, is his eye just the outward shape of an eye? Or does he have a lens, and a retina, and an optic nerve, and a brain made of neurons processing images?

Answer

When Odo assumes a humanoid form, he has no detectable brain. From the DS9 episode "Dramatis Personae":

ODO: Doctor, I believe the Klingon brought the matrix with him and everyone in Ops was affected except me. Without a humanoid brain, my system rejected it.

If there is no brain, then there are no nerves, optic or otherwise.

For another statement about the lack of nervous systems — but from Beta material — we have the following excerpt from Deep Space Nine Rebels: Book One by Dafydd ab Hugh:

Founders — Changelings — didn't have a distinct central nervous system or brain, of course...the Founders' mental activity occurred everywhere and nowhere.

Concerning other organs, we have the following statement from Odo in the episode "The Forsaken":

ODO: I don't eat. This is not a real mouth, it is an approximation of one. I do not have an esophagus or a stomach or a digestive system. I am not like you.

(Thanks to @ThePopMachine for digging this one up!)

Taking all of this together, it would seem that Odo, even when taking humanoid form, has none of the organs that a humanoid would normally have. What we see on his exterior is for appearance's sake only.

Of course, if there are no physical manifestations of a nervous system within a Changeling who is infiltrating a humanoid community, then a such an individual could be caught by a rather simple medical scan. This would suggest that skilled Changelings ought to be able to simulate not only the internal appearance but also the electrical activity of a nervous system, in order to pass a basic tricorder test for being a humanoid. (This must be the case, since we know from several DS9 episodes that Changeling impostors are very difficult to detect.)

We also know that shape-changing is taxing on a Changeling. It would stand to reason that when a Changeling is not trying to fool solids (e.g. Odo on a normal day), then a Changeling would probably not be simulating a nervous system, given the complexity of the feat. Also, it may be that only very few Changelings have the endurance and skill required for it.

All in all, we can safely asssume that, when Odo assumes his humanoid form under normal circumstances, he has no brain, nerves, or organs, and his "eyes" are just for appearance's sake.