Nothing, that is, except their genes. When researchers like Mark Siddall started sequencing myxozoan genes in the 1990s, it became clear that these oddballs are probably animals, and likely cnidarians. Paulyn Cartwright confirmed that last year by comparing the full genomes of five myxozoan species. She showed that their closest relatives are indeed cnidarians—and specifically the jellyfish, box jellies, and their kin.

The myxozoans probably split away from these other cnidarians over 500 million years ago. During the intervening aeons, they have lost almost all the features of their former selves. As science writer Jennifer Frazer once wrote: “Once upon a time, a jellyfish became a parasite, and its descendants became unrecognizable.”

The details of that process are still unclear, but one living animal might shed some light on it. It’s called Polypodium hydriforme, and it’s the closest living relative of all myxozoans. It infects the eggs of sturgeons and paddlefish; as Cartwright says, it’s a caviar parasite. Within each egg, Polypodium develops into a bizarre colony of inside-out larvae—each has a ‘gut’ that sits within its ‘skin’, and several of them are connected in a single long sac. Once the eggs are released into the water, the larvae invert their bodies and break apart from each other.

In this free-swimming form, they look very jellyfish-like, with identifiable tentacles, mouths, and guts. Perhaps the ancestors of myxozoans went through a similar phase in their evolutionary history, when they were already devoted parasites, but still kept some obvious traces of their cnidarian heritage.

As they evolved further down the parasitic path, they lost these ancestral physical features. They did away with many genes too. “They have the smallest known animal genomes,” says Cartwright, “and they lack some of the genes that we consider hallmarks of animal development.” For example, the all-important Hox genes, which direct the construction of animal bodies and which I wrote about last week, are simply missing in myxozoans.

However, these parasites have kept the genes that cnidarians use to build their stinging cells; they now use those to build the polar capsules instead. “They’ve adapted the stings for attaching to their hosts,” says Cartwrght. “But everything else that makes you a proper multicellular animal was lost.”

Who would have thought that jellyfish, of all creatures, would have evolved into a dedicated parasite? And yet, these animals are hardly alone. An estimated 40 percent of animal species are parasites, and a recent study estimated that parasitism has evolved among animals on at least 223 separate occasions. Wasps and flies have done it. Worms and flatworms have done it. Fish and barnacles have done it. Even jellyfish—or, at least, jellyfish-ish creatures—have done it.