Nematodes are usually vermiform, long and slender, but some species are swollen. Most people know them as roundworms because their cross-section is round. They range in size from .08mm to 7m.
Nematodes are a tube within a tube.
The outside tube is the body wall which consisits of muscle layers that are used as a protective covering. The inside tube is the digestive system. The esophagus is the front of the digestive tube. Nematodes exist almost everywhere, including soil, plant and animal surfaces and interiors, decomposing life forms, and fresh and salt water and their subtending sediments. Nematodes can be grouped according to what they eat.