线虫门, or Nematoda, is a large phylum of roundworms that are found in various environments, including soil, freshwater, and marine habitats. They are characterized by their elongated, cylindrical bodies and lack of segmentation. Many species are microscopic, while some can grow to several meters in length. These worms play essential roles in ecosystems, such as decomposing organic matter and serving as food for other organisms. Some nematodes are beneficial for agriculture, while others can be harmful, acting as parasites in plants and animals, including humans.