Worms can be obtained while digging, occasionally in stumps and in compost bins while processing material. Required by Concordat of Worms and can be used as simple bait for fishing.
In the update "Under Pressure" (Thu Apr 11, 2013) Worms became a crucial part of increasing Humus purity.
When a Worm is placed inside a container it will attempt to consume Any Plant from the inventory squares immediately next to it. This will occur once every 12 RL hours. This timer restarts from zero every time you move the worm.
Each time the worm eats, it will gain ~10% of the difference between its own purity and that of the food it ate.
Worms also increase the "Worm efficiency" of Compost bins. Having less than 100% Worm efficiency will always reduce the purity of humus produced.