Nemertea, nemerteans or ribbon worms

Nemerteans are otherwise known as ribbon worms, proboscis worms or more ominously, smiling worms. They are usually aquatic and predatory and can be anything up to 30 metres long. However, there are a few southern hemisphere species that are relatively tiny and fully terrestrial, like these ones. And they are a little strange.
All nemerteans have an eversible proboscis, which is definitely one of the grosser things to watch with a magnifying glass. At rest, the proboscis lies in a cavity along almost the full length of the body. When attacking prey, the front end of the animal instantaneously everts an enormous white mass of venomous, sticky mouthparts, completely covering the possible food. It can be retracted almost as quickly. 

All nemerteans travel on a trail of their own mucus, making them initially easy to confuse for a slug or a flat worm as I did once, getting the shock of my life when it everted all over a woodlouse.
They also share an ability with the flat worms to have a body that can thicken and shorten when disturbed or at rest, and telescope and become thinner when moving. They can also have hundreds of the light-sensitive simple pigment-cup eyes, like the flatworms.

As exotic species, they have been transported into the northern hemisphere by human activity. The UK has at least three species, probably from Australia originally. First observed around twenty years ago, they are rapidly expanding their south-west and southern range.