This is a close-up picture of a hagfish's slime. The hagfish can produce vasts amounts of slime, enough to fill a bucket that they happen to be captured in. This slime consists of fibrous protein threads, mucus, and seawater. You can see the threads in this picture, if you look carefully.
Recent research suggests that the bulk of the mucus is actually seawater that is trapped in the mucus-coated threads . This makes a certain amount of sense, given that the mucus expands rapidly when in contact with water (much like a sponge). It may also explain how these hagfish can produce such vast quantities of slime...basically, they are just puffing up a little bit of slime with water.
What hagfish use this slime for is still under debate. Most seem to be leaning towards the idea that the slime can clog the gills of potential predators.