Jellyfish don't have a brain.

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Most animals have some sort of centralized nerve center, aka a brain, although many have only simple versions called ganglia – concentrations of nerves that control other nerves around them. Jellyfish have no such central place; in fact, they have two nervous systems. A large nerve net controls swimming and a small nerve net controls all other behaviors, including feeding and spasm response (briefly curling into a ball). This body-wide network of small nerves somehow makes it possible for a jelly to figure out where the different parts of its body are and to act accordingly – for example, using a single tentacle to move prey to its mouth.

The large nerve net includes rhopalia, finger?like structures on the edge of the jellyfish’s bell. These contain crystals that give jellies a sense of up and down, much like those in our inner ear, and a small pigment spot that may sense light, chemicals, or some combination of the two. Each rhopalium helps coordinate the pulsing motion of normal swimming and, not surprisingly, are mostly located near swimming muscles.
“Jellies are like the original computer networks, with little servers all along the margin of their body that they use cooperatively,” says Rebecca Helm, a researcher at Woods Hole Oceanographic Institute. “They have a net of cooperative nerve bundles that talk to each other and some pockets of centralized nerves, but no master controller. That’s nice when, say, a sea turtle bites off part of the bell. That isn’t the end of everything, because jellyfish can lose some of those servers. It can function with, say, seven rhopalia rather than eight.”

Scientists can only speculate why jellyfish didn’t evolve a central nerve center or brain. “Their ancestors branched off to one side of the tree of life, so it could be that evolution of a centralized nervous system occurred really early on the branch humans are on, but jellyfish kind of got stuck,” says Helm. She recently published a paper pulling together information on the evolution and development of Scyphozoans, the taxonomic class that includes jellyfish most familiar to beach-goers, including moon jellies, sea nettles, and lion’s manes.

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