To my intense disgust, we have these things in the garden.
They are a type of parasitic land flatworm that preys on earthworms. Don't know which species this is, but it is probably of the genus Bipalium. They capture an earthworm and evert their esophagus to digest the poor creature. Looking through the intertubes, it appears that an infestation of these things can clear out a compost worm-bin in just a few days. On one hand, we're lucky as we don't have a compost worm-bin. On the other hand, it looks like we'll never be able to have one now.
We see them because they come up out of the damp soil and on to the tiles when the sprinklers run. They make good speed, leaving a bit of a slime trail like a slug. They can be almost a foot long, and the distinguishing features are the hammer head and longitudinal stripes. Originally from Asia, they've made their way all over the place in the pots of nursery plants. Luckily they are quite stupid, and most of them die on the tiles without ever getting back to the damp earth. Apparently there's no point in chopping them up, as like other planarians, they regenerate whole bodies from chopped bits.
All in all they're disgusting and any ideas on how to get rid of them would be appreciated.