Hagfishes can produce huge quantities of mucus! A 50 cm long hagfish can fill an 8 litre bucket of water with slime in minutes.
Here are a number of interesting videos online about Hagfish slime.
1) Great video of Eddie Kisfaludy at Scripps Institution of Oceanograpy showing hagfish 'behind the scenes'
2) 'Electrically induced sliming':
3) Hagfish and the Disgusting Slime. This video shows the hagfish's ability to produce slime 'on demand'.
The fishes in the videos are not always well treated. I don't condone mistreating any fish, but the videos are certainly educational.
Hagfishes have 70 to 200 portholelike structures on the side of the body. These are the mucous glands. The slime glands contain mucous cells and thread cells, the latter strengthening the mucus. The mucus consists of a protein and carbohydrate that binds to the water and expands to produce the slime.
The production of mucus is believed to be a defence mechanism.
In case you are wondering, yes, hagfishes are fishes. Check out the What is a fish page.
Helfman, G.S, Collette, B.B., Facey, D.E. and B.W. Bowen. 2009. The diversity of fishes: biology, evolution and ecology. 2nd edn. Wiley-Blackwell, Chichester. Pp. 720.