Pill millipedes belong to the class Diplopoda. Their trunk segments are formed from fused pairs, which is why most bear two pairs of legs. Pill woodlice are isopod crustaceans, relatives of shrimp and crabs, with one pair of legs per segment and two pairs of antennae. The shared ability to conglobate, to roll into a sealed sphere, evolved independently in each lineage.
The convergence is functional. Rolling protects the soft underside and legs, presents only hardened plates outward, and reduces water loss, all valuable to a small litter-dwelling animal that cannot outrun anything. Pill millipedes generally roll into a tighter, smoother ball with fewer visible segments, and have a single pair of antennae.
Despite the name, almost no millipede approaches a thousand legs, and pill millipedes have among the fewest of all. The single exception is Eumillipes persephone, an Australian soil-dwelling species described in 2021, one specimen of which carried 1,306 legs - the only millipede yet recorded above a thousand.