How balls of blackworms avoid the knotty step

MANY ANIMALS find safety in herds, colonies, schools or swarms. But few species opt for the technique of the stringy, water-dwelling blackworm Lumbriculus variegatus, a creature that at a few centimetres in length is far longer than it is wide. In trying times, for instance when water is scarce, tens of thousands of them can swiftly wriggle together into a tangled ball, to seek a wetter environment and save from desiccation all but the poor creatures on the blob’s periphery. How they escape that situation, however, had until recently been a mystery both to biologists and to anyone who knows about knots.