Hagfish "eyes" are so primitive that it's debatable whether they can be called eyes in the proper sense. People have some very mean things to say about hagfish vision:
- ★★★☆☆ Eyes (Hagfish Biology chapter 5)
- ★★☆☆☆ "Eyes" (Hagfish Biology chapter 5)
- ★★☆☆☆ Eyespots
- ★☆☆☆☆ starkly rudimentary eye (https://www.biorxiv.org/content/10.1101/2020.08.24.265124v1.full)
- ★☆☆☆☆ rudimentary hagfish retina (https://www.biorxiv.org/content/10.1101/2020.08.24.265124v1.full)
- ★☆☆☆☆ hagfish eyes are strikingly rudimentary (https://pmc.ncbi.nlm.nih.gov/articles/PMC8826474/)
- ★☆☆☆☆ poorly developed visual [...] systems (https://karger.com/bbe/article-abstract/48/5/262/42399/The-Sensory-Biology-of-the-Living-Jawless-Fishes-A?redirectedFrom=fulltext)
[HAGFISH EYE STUFF]
But it wasn't always this way! Probably. There is some support among hag-heads for the idea that hagifsh once had more developed eyes and simply evolved simpler eyes - after all, there isn't much use for vision when you're in the deep sea! Evidence is limited though: hagfish have a teensy-weensy fossil record (see Fossils), and eyes don't fossilise well to begin with (https://pmc.ncbi.nlm.nih.gov/articles/PMC8826474/#B17).