The preceding discussion focuses on bilaterally symmetrical animals — those with a right and left side, a head, and a tail end. Most animals are bilaterally symmetrical: you, your dog, your goldfish, your hamster, and your houseflies.
But a few animals aren't symmetrical — jellyfish, for example. As it turns out, jellyfish also have genes related to the Hox cluster. They're much different from the genes that humans share with mice or even flies, which makes sense, because the hypothesized common ancestor of the bilateral animals and the jellyfish lived millions and millions (and millions) of years ago.
Now, a jellyfish doesn't have a head in the sense that humans do, but it does have a place where the mouth is, which is probably as close to a head as you can imagine. Jellyfish have an oral side and an aboral side — in plain English, a mouth side and a side opposite the mouth side. Genes related to the Hox genes in bilateral animals are responsible for this oral-aboral patterning, and in honor of the group that jellyfish find themselves in — the Cnidarians — these genes are called Cnox, instead of Hox, genes. (The C is silent; it's just there to mess with you.)
Cnox genes are similar to the ones involved in head formation in bilateral animals. Scientists can investigate the function of these genes in jellyfish by altering them genetically. One such experiment resulted in a jellyfish with multiple heads and multiple functional mouths!
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