Still looking for a cite on the cyclops jellyfish.
From JellyfishFacts.com
For years now this Cyclops, by Redon, has been one of my favorite paintings. I use a close up of the gentle beast as one of my avatars in another site–I too, have only one (working) eye, and perhaps can be so affable.
I also thought when posting about the “force of the bilateral” in morphology.
So I went to the wiki “Cyclops,” which led me to …
[WARNING: every cite following, except the NIH, has nightmarish photos of human deformity/teratology. Thanks *a lot, *SD.]
… the fascinating entry Cyclopia, where the real biology is discussed, which led me to Holoprosencephaly. I then found this on the developmental biology.
From the NIH is a genetic overview.
OK, having found the actual page where you got that from (would providing an actual link have killed you?), I am going to state, with a fair degree of confidence, that whoever wrote it has no idea what the hell they are talking about. It is a poorly digested, ill explained, mish-mash of (at best) half-understood and perhaps sometimes misremembered facts. For one thing (amongst many examples of nonsense), it implies that all jellyfish have lensed eyes, when, as I noted, only box jellyfish do, and then asserts that these eyes are useless to the jellyfish!
First, I’m sorry I forgot the link.
Second, I was looking for a better cite on jellyfish being farsighted (I found a few pages, none of them credible enough for me) and I realized something. Why in the nine hells should I take the word of a stranger over the word of a fellow Doper? If there’s anybody you can trust to know all about jellyfish vision, it’s a Doper.
I still can’t find the thread devoted to the one eyed jellyfish. It’s driving me slightly more insane.
And some scallops have many small “eyes” all around the edge of the shell. I use quotes because I don’t remember how optically efficient these organs are–whether well-formed like the eyes of most birds and mammals, or just vaguely able to perceive changes in the ambient light.
Hey, I don’t claim to be an expert on jellyfish, but that page (probably the site as a whole, but I didn’t look closely at much else) just did not have the ring of quality information to me; I would much sooner trust what I found on Wikipedia. I am not saying that its facts are necessarily wrong, but they are not perspicuously organized and sometimes quite misleadingly stated. I get the impression that the site is some sort of content farm, presumably trying to bring in advertising bucks, and not written by a jellyfish expert or even a jellyfish enthusiast. Its real interest is human eyeballs (although I find it hard to believe that much money is to made out trying to sell stuff to people looking for facts about jellyfish!).
It may well be true, as he says, that the lens of a box jellyfish eye does not have the right focal length to produce an image on its retina, but the conclusion should not be that the animal is farsighted, because its retinal image is always badly out of focus, but that the “lens” isn’t really there to produce an image at all (perhaps it is not even really there to focus light). I do not know much about jellyfish, but I do know enough about vision to understand that the importance of a retinal image to vision is much overrated. Many animals (including most jellyfish) have very useful eyes that do not have any any focusing mechanism, and do not produce an image at all. Nevertheless, their eyes guide their behavior and help to keep them alive and maybe to find mates. As that page (rightly) says, a jellyfish does not have a brain, and its nervous system is very simple, so even if its eye did produce an image, it does not have enough onboard computing power to extract any useful information from it.
Actually, I think there would be a lot of advantages.
Imaging having an eyeball at the end of an elephant-like trunk. Or maybe even better— an eye on the tip of your index finger. You could point at anything and see it–even it was hiding around a corner, or inside an underground burrow, above your head or behind your back…
This would be very useful for gathering food, hunting animals that are hiding from you, or seeing a predator sneaking up on you from behind.
( It would also be very useful for working on your car engine when you can’t quite see that damn bolt you have to unscrew.Or for your dentist, who wouldn’t have to hire an assistant just to hold a mirror at the back of your mouth… 
)
Of course, a single eyed creature would lose the benefits of parallax and judging distance…but that is easy to overcome by learning. (People with only one eye can still drive and function well, because their brains have learned to judge distance from other visual clues.)
Or for becoming a Batman villain, apparently…
The fact is that even most two eyed animals, except for primates and predators, probably don’t, or even can’t use binocular depth cues. Many have eyes on either side of their head, giving a wider overall field of view, but hardly any overlap between those of the two eyes. Even two eyed humans (and those other primates and predators) probably rely on binocular vision to judge distance far less than is popularly thought, and make greater use of those other depth cues you mention. This is not really something that has to be learned (except, perhaps, in infancy, at which time we probably have to learn to use binocular depth information too) because we all unconsciously use such cues all the time. Probably the most important source of distance information in most circumstances is the parallax produced by self-motion; even quite small movements of the head, which we usually make quite unconsciously, can provide significant and accurate depth information.
Yes, this is why the nightmare creature has its eye on a proboscis. It waves it back and forth as it chases you, the better to judge its distance , AND STAY ONE STEP BEHIND YOU FOR THE WHOLE NIGHTMARE!!!
Surely, nobody is suggesting that the marine biology presented by Stephen Hillenburg is less than 100% accurate?
Not so inaccurate. Sheldon would seem to be modeled on the planktonic copepod Cyclops mentioned in post #6 (although that’s a freshwater genus).
Very droll, that page;