Why did the chiton roll into a ball?

“To get to the other side,” said Julia Sigwart, an evolutionary biologist at Queens University Belfast in Northern Ireland.

About 500 million years ago, a couple species of now extinct trilobites became the first animals to roll themselves into a ball for protection. The trilobite’s living doppelgänger is the chiton. This marine mollusk’s plated shell drapes over a soft body and mucousy foot, giving it the appearance of a flattened piece of shrimp nigiri. But you’re more likely to find it clinging to a tidepool rock than awaiting chopsticks on a dinner plate.

Like trilobites, three-banded armadillos, pill bugs, hedgehogs and other animals, the chiton can roll itself into a ball. Many scientists had assumed this acrobatic maneuver, known as conglobation, defends the animals, most of which are smaller than an inch or two, against predators. But Dr. Sigwart, who studies chitons, never really bought that explanation: If a predator can swallow you whole, she reasoned, rolling into a tic-tac probably would not save you.

In a series of lab experiments, Dr. Sigwart showed that rolling into balls has more to do with helping chitons get to places where they can reattach after losing their footing. She hopes that these findings, published Wednesday in Biology Letters, add evidence to a novel argument: that chitons are capable of making decisions.