Growing up, Chelsea Wood dreamed of becoming a marine biologist and studying sharks or dolphins—the kinds of big, exciting animals that biologists call charismatic megafauna. Instead, during a college internship, she found herself peering through a microscope at a snail’s innards.

The snail was one she knew well. As a kid, she had often plucked Littorina littorea periwinkles off rocks along the shores of Long Island and dropped them into buckets to watch them crawl around. But she had never seen inside one. She cracked a snail open, teased out the soft parts, and under her magnified gaze saw “thousands of little white sausage-shaped things dumping out of the snail’s body,” she says.

The sausages were the larvae