While jellyfish do have a nerve-net (a series of nerves throughout their body linking their senses to their motor system), they do not have a brain. This means that they are able to respond to external stimuli through a sense of touch, but the mechanism by which they respond is more like your knee getting tapped and making your leg jump, than it is a complex response to pain which may go both directly via a fast reaction (which helps you to take your hand off the hot object before you know that it’s hot) as well as a slower response which tells you that you have to do something about the molten skin on your hand. The latter is what we should think of as pain, as a felt and conscious response to a negative stimulus. Because a jellyfish does not have a brain, there isn’t (as far as we are aware) a thing that it is like to be a jellyfish in pain (see Nagel).

Plants do have methods for detecting changes in the environment, but they are not through nerves. Plants can detect changes in light, through chemical responses which trigger hormones to be released, and they can detect gravity, so that they know which way to grow. They can even respond to pressure (thigmotropism) which may be caused by wind or touch. The way the plant moves in response to this is via hormonal changes. There is a lot written about plant response to stimuli here.

So jellyfish don’t seem to have pain response because there isn’t a brain to do the feeling, and plants don’t, because there isn’t even a nervous system. It is easy to anthropomorphise the fact that both will react to move away from negative stimuli, but this is a helpful evolutionary adaptation which comes about to help the organism survive.

Jellyfish not only experience loss, but they can do something about it. They are able to regenerate lost limbs using a process called “symmetrization“. However, whether they feel the loss of a friend, a lover, or a favourite movie star is yet to be written in the annals of jellyfish lore. This is probably because jellyfish are famously bad at spelling.