A web site for fans of earthworms tackled the question recently:

Yes, it is now accepted that worms feel pain – and that includes when they are cut in half.

They do not anticipate pain or feel pain as an emotional response, however. They simply move in response to pain as a reflex response.

They may curl up or move away, for example, from painful or negative stimuli.

Aimen Mirza, “Do worms feel pain? (Can Earthworms Sense Painful Stimuli?)” at WORMMY (October 12, 2021)

Possibly in line with the growing support for panpsychism in science, University of Washington evolutionary psychology professor David P. Barash, asks us to consider that worms do indeed feel pain in a deeper sense than an automatic response:

I vividly recall, as a child, watching with horror as my uncle threaded a worm on a hook. The victim wriggled with what in a human would unquestionably be agony, while my uncle reassured me, “It’s not feeling pain.” As an adult researcher (who should have known better), I’ve seen snakes, fish, and cockroaches spasm when subjected to electric shocks.

David P. Barash, “Even Worms Feel Pain” at Nautilus (March 2, 2022)

First, let’s look at his argument. Barash, author of many books including Through a Glass Brightly: Using Science to See Our Species as We Really Are (2018), alludes to a growing awareness that our traditional evolutionary assumptions about sentience may not be correct:

Who feels more pain, a person or a cat? A cat or a cockroach? It’s widely assumed animal intelligence and the capacity to feel pain are positively correlated, with brainier animals more likely to feel pain, and vice versa. But what if our intuition is wrong and the opposite is true? Perhaps animals that are less intelligent feel not only as much pain but even more.

A correlate of this attitude, rarely challenged even today, is that the more similar animals are to us, the more likely they are to feel pain. And in proportion as they are “simple”—i.e., stupid—they can’t. I want to take issue with this and suggest a counterintuitive hypothesis: That animals with less cognitive capacity might feel at least as much and perhaps more pain than their smarter cousins.

David P. Barash, “Even Worms Feel Pain” at Nautilus (March 2, 2022)

Although Barash does not say it in so many words, we have tended to assume that invertebrates did not evolve to be as intelligent as vertebrates and that cold-blooded (exothermic) life forms did not evolve to be as intelligent as warm-blooded (endothermic) one. But, as we’ve noted earlier, recent research has shown that some invertebrates, like the octopus, are in the range of intelligence we associate with mammals and birds, which means that they may well experience pain as suffering. One result is that some countries now require humane methods of slaughter for such invertebrates, as well as for (cold-blooded) fish.

On the other hand… there is no evidence that most invertebrates are especially intelligent. So we are really looking at a puzzling question as to how some, but not most, invertebrates and exotherms came to have comparatively high intelligence and thus (probably) more complex sentience.

And now to the worms. Barash offers an evolutionary argument that “animals with less cognitive capacity might feel at least as much and perhaps more pain than their smarter cousins”:

Insofar as it is a crucial alarm signal, pain should be a cross-species universal, no less valuable for paramecia than for people. I agree with the argument made by Richard Dawkins in his book Science in the Soul, in a chapter titled “But Can They Suffer?,” that smaller-brained creatures just might have greater need for this signal. “Isn’t it plausible that an unintelligent species might need a massive wallop of pain, to drive home a lesson that we can learn with less powerful inducement?” Dawkins asked.

David P. Barash, “Even Worms Feel Pain” at Nautilus (March 2, 2022)

Barash reasons that a less intelligent creature would need more pain than a more intelligent one, in order to teach it to avoid risk and harm:

The dummies, accordingly, would benefit more than the smarty-pants from an especially potent stimulus, a blast of something deeply unpleasant—call it “pain”—more likely to evoke whatever passes for memory and learning in their admittedly dim minds. If so, then they would benefit from a particularly loud alarm bell: More pain rather than less.

David P. Barash, “Even Worms Feel Pain” at Nautilus (March 2, 2022)

Thus he concludes, “Pain would have been among the most fundamental traits to have emerged.”

Given the adaptive value of pain, that sensation would not only be conserved over evolutionary time, but ancestral, among the earliest and most fundamental traits to have emerged. This makes it, well, pretty much insufferable to deny other animals the experience of pain that we know all too well.

David P. Barash, “Even Worms Feel Pain” at Nautilus (March 2, 2022)

Wait. Barash’s hypothesis overlooks the fact that suffering is more than an alarm system. An alarm could be going off in an empty building.

If a fire alarm went off in an empty building, a built-in fire safety protocol might seal all the windows, turn off some systems, and send a message to the firehall, copying the maintenance crew’s mailbox, without anyone experiencing anything. Life forms are fully capable of much more complex responses than these, even without any apparent self-awareness. Tests for the self-awareness that would cause them suffering cannot depend only on identifying a response to pain.

To “feel” pain, a life form must have a unified self, that is, be a subject of experiences. We all know that a dog experiences pain or rejection as happening to him as the subject of the experience. But does an earthworm experience pain that way?

Essentially, experiencing pain (sentience) can mean either of two things: the ability to react to pain or the perception of the pain as happening to one’s unified self. Or both.

In the case of, for example, the octopuses, their learning skills and ability to form some relationships tipped the balance in favor of assuming that they experience pain as selves.

But earthworms? Garden slugs? Tent caterpillars? If some invertebrates show much more individual intelligence (and capacity for suffering) than we had expected, it hardly follows that all do.

We don’t make those assumptions about vertebrates. We don’t just assume that, because some chimpanzees can learn hundreds of signs in sign language, therefore most raccoons can do so as well. We risk trivializing the question of suffering — and impeding humane reforms — if we cast the net too widely.

And getting it right is important. We ought to err on the side of cautious acceptance where there is reasonable evidence. For one thing, life forms with some intelligence but not that of an adult human may in fact experience more pain because, while they may experience their environment as selves, they have none of the buffers against pain that abstract reasoning offers. That is probably the case with, for example, human infants as well as with many animals, vertebrate or not, that are sentient in both of the senses above.

It’s worth noting that evolutionary theory hasn’t really provided us with much guidance as to what to expect re intelligence. That means much more research is needed. But then the researchers won’t likely mind that.

You may also wish to read:

Can crabs think? Can lobsters feel? What we know now. In Switzerland, it is now illegal to boil a lobster alive. Are the Swiss right? Is it cruel? How does a self that feels pain come to exist? And how do we distinguish information use — computer style — from self-awareness?

and

Abortion advocate admits in a medical journal that unborn children feel pain. The scientific community has for decades misrepresented the straightforward science of conception and fetal development for ideological reasons. I have cared for hundreds of premature infants and it is very clear that these very young children experience pain intensely. An innocuous needlestick in the heel to draw small amount of blood would ordinarily not be particularly painful for an adult. But a tiny infant will scream at such discomfort. (Michael Egnor)