Nematodes are small roundworms classified into 5 groups based on their food source. They have bilateral symmetry, a pseudocoelom body cavity, and generally reproduce sexually. Nematodes transport food through their pseudocoelom and digestive system, and breathe and circulate fluids through diffusion and a circulatory system of tubes. Many nematodes are parasitic and can cause diseases in humans and other animals, while others are beneficial as they eat pest insects.