An elusive worm: the Salinella is shrouded in mystery

A 19th-century zoologist found the little salt dweller', which could be a portal to the past - if only we could locate it again

Last February, with colleagues Gert and Philipp and my daughter Francesca, I made the long journey to an unremarkable city called Rio Cuarto, east of the Argentinian Andes. We went in search of a worm of unusual distinction.

Why a worm? As humans, we naturally love the animals that are most familiar. But from a zoologist's point of view, the vertebrates, from mammals and birds to frogs and fish, can be seen as variations on a single theme. We all have a head at one end (with skull, eyes and jaws); in the middle, a couple of pairs of limbs (a goldfish's fins, or your arms and legs); and, holding all this together, a backbone ending in a tail.