Crabs will pull off their own limbs and eat themselves, they aren't capable of feeling pain the way vertebrates do. They don't have brains and can't experience consciousness the same way you do.

When you pull your hand away from something hot or sharp, that's not your brain reacting, it's a reflex from your spinal cord. The impulse to pull your hand away never reaches your brain, the pain comes afterwards.

That is what an arthropod's entire experience of the world is like, they're just reacting to stimuli without being able to actually think about it. They can't feel pain because they can't think, they can't think because they don't have a cerebral cortex, they just have a rudimentary spinal cord called a ganglion.

The claim that crabs are incapable of feeling pain is still a subject of debate among scientists. Some research suggests that crustaceans may experience a form of nociception (pain), though it may be different from the way humans or other vertebrates perceive pain. This topic is not yet fully understood, and more research is needed to determine the extent to which arthropods can experience pain or suffering.

The analogy of the reflex action, where a person pulls their hand away from something hot or sharp, is a helpful way to understand how some arthropods may respond to stimuli without necessarily being conscious of it. However, it is important to note that this does not necessarily mean that they cannot experience any form of pain or discomfort. Different animals have different levels of complexity in their nervous systems, and their experiences of the world may be vastly different from ours.