Nematodes are grouped by their trophic group or basically what they choose to eat: fungi, bacteria, algae, nematodes, protozoa, insect larvae, and plants. These groups are often microscopically identified by their mouth-piece. These mouth parts serve the nematodes well. Bacterial feeders have a tube that can suck up bacteria much like a vacuum. The parasitic or omnivores have a "stylet" or a hollow piercing mouth part that can puncture through the plant and allows the nematode to withdraw plant tissue for consumption.
a) bacterial feeder, b) fungal feeder, c) plant feeder, d) predator, e) omnivore
Figure credit: Ed Zaborski (University of Illinois)