This is probably the most rational post so far.
Veganism is commonly defined as not consuming animals or animal products, but that's a simplistic definition.
If tomorrow we found a plant that had a brain and could experience pain, according to the simplistic definition, it would be okay to eat. But most vegans would have a problem with this.
It seems to me that a more accurate definition of veganism would be that vegans seek to minimize suffering by sentient beings. So plants with a brain would still be wrong to eat.
But what of animals without brains? It seems that most vegans don't have a problem with the countless deaths of non-sentient, single-celled animals that happens all the time in the modern, developed world. (And quite frankly, I prefer to not have parasitic protozoa in my tap water, thank you very much.) So there's already a precedent. And even microscopic multi-celled animals appear to have no sympathy from vegans.
Then again, a microscopic organism is obviously not sentient in any way we could possibly be familiar with, even if a few of them (such as hydras) have a rudimentary nervous system called a "nervous net". Due to how primitive these nervous nets are, and the size of the creatures, it's unlikely they are sentient. Their nervous systems are strictly reactionary.
What about larger animals?
For macroscopic animals, sponges seem to be obviously lacking sentience. They lack a nervous system. (Hydras have a better nervous system than sponges.) I'd argue that killing a sponge for personal use is obviously vegan, even though they are technically animals. They just have no capability of being aware of pain.
In an increasing order of complexity, creatures like jellyfish come next. This is overall biological complexity were talking about - jellyfish lack a digestive system or circulatory system. But they do have a neural network. It probably can't do much. If I had to bet, I'd bet they aren't sentient either, but I'd like to do more research.
Bivalves also appear to fall into this group, although they evolved from more complex animals. Bivalves are, after all, mollusks, which include such very intelligent non-vertebrates as squids. But while squids were developing in an environment where evolving a large, complex brain mattered, bivalves were adapting to an environment where a brain was redundant. Oysters, mussels and scallops lack a true brain, instead having clusters of nerve cells (ganglia) in various parts of their body to control various things. Again, like jellyfish, they seem to lack sentience, but I'd like to do more research.
So as a vegan, I'd say sponges are a-ok, while jellyfish, comb jellies, mussels and oysters are more likely than not to be okay. But considering that we don't need to eat jellyfish, comb jellies, mussels, or oysters, and there doesn't seem to be great research on their nervous systems, I'd avoid them until we know more. Although I could see a strong argument being made that oysters *are* vegan.
While it would be pointless to avoid consuming sponges since they are no more capable of feeling pain than plants are. (Then again, how often does the average person buy something made from a sea sponge?)
Admittedly, regardless of the species we use and consume, how that species is cultivated or harvested needs to come into play. If how that species is collected greatly damages the environment, I'd argue it would be non-vegan, due to the indirect but predictable suffering that would happen to sentient animals.
Anyways, this was a fun post to write. (I had to look up quite a few things that I haven't studied for probably close to two decades. :))
I'll conclude by saying that we, as vegans, should avoid having our beliefs rigidify into dogma for dogma's sake. Most of us are veg*n so that we may reduce the suffering of sentient creatures. Lets remember that. And lets be always willing to debate. :)