Many nematodes are parasitic organisms which live inside the guts of many animals, gastropods and fish ... perhaps even you. They grow up to 5mm in length and consist mostly of reproductive tissue. They feed directly off its host by attaching themselves using hooks and sucking up food through a structure known as the stylet.
Other nematodes feed on bacteria, fungus or plants, and can cause much havoc to herbs and fruit (such as the tomato) by reducing the flow of water through the plant and depriving it of nutrients.
May be used as an insult.
For some time in the Northern Outlands, the act of preventing drunken friends from doing stupid things was often summarized as, "Friends don't let friends sleep with nematodes."