tonmo said:
I'll be very direct with my question:
In the context of evolution, can someone explain to me how a squid's eye (esp. Architeuthis) can so closely resemble human eyes?
Am I correct in that Architeuthis has an "eyelid"? Or am I making that up?
Wondering why some sea creatures with eyes have eyelids and some don't.
I have no idea about eyelids, so I'll leave that to someone else.
The similarity between eyes is frequently referred to as a prime example of "convergent evolution." The general idea is that the survival benefits of having an eye with particular features are so significant that any adaptation that leads to those features will be selected for very strongly, so even when two species have arrived at the basic plan for an eye (or whatever) through independent evolution, there will be a very strong pressure to reach a similar morphology, because that morphology conveys a very definite advantage.
Although cephalopod eyes' gross morphology is similar to vertebrates, there are a number of salient features that point to their independent origins. Vertebrates have "inside-out" retinas, while cephalapods have more sensible ones. Vertebrates tend to have rods and cones (or similar) packed onto a roughly hexagonal grid arrangement, while cephs have sort of rectangular photoreceptors (rhadomeres or something like that) packed into square-ish grids. There is some difference in the focusing arrangement of the lense as well.
All this makes it interesting to ask what is "random feature" versus "actually important for eyes"-- it seems likely that the lense, pupil, and basic retina arrangement is so effective that it's been developed independently in cephs and verts, while some of these other aspects are "any way you do it works fine," so they developed independently and were preserved.
There is another major factor, though-- eyes of some sort were apparently part of the base body plan-- some of the most surprising results of the early HOX gene research was that creatures as different as fruit flies and people had the same genes code for eyes, so eyes of some primative sort developed in the common ancestor of all of the bilaterally symmetrical critters (and it looks like perhaps some of the radiates as well). So, there may be some aspects of the primative eyes of our common ancestors that biased the development of advanced eyes in some particular direction.
It's particularly interesting to look at the eyes in nautilus, because they seem to be much more primative than most ceph eyes, yet they have some of the essential features of modern eyes, like a pupil of sorts, but not a proper lense or a sealed eyeball. I don't know of any examples in the vertebrates that are "living fossils" that show the "missing link" between primitive eyes and the modern eye structure.
Come to think of it, I wonder if there is any primitive eye in the sea squirts that are primitive chordates-- since they are often touted as representative of the ancestral vertebrates, and it's known that vertebrates and arthropods (and I assume cephalopods) evolved from a common ancestor that had the same homeobox gene controlling eye creation, it seems like sea squirts should have either eyes or some evidence that they lost their eyes based on their sessile lifestyle or some such.
, but I'm interested in what the ideas are behind how two creatures from completely separate evolutionary paths could share such similarities in something as complex as eyes. 
