Nematodes are most successful as parasites, a lifestyle they have evolved several times in their long history. Nematodes that infected the ancestors of whales have evolved with them, becoming some of the largest parasites on the planet. Nematodes that infect freshwater stingrays in the Amazon are most closely-related to Pacific species, corroborating the body of evidence showing that the Amazon once flowed to the West. There are even nematodes that infect other nematodes. The body plan these worms have used to be so successful is a simple one.
Nematodes have a complete gut, a flexible outer cuticle, but few other external features.
They do not have eyes, legs, hairs, scales, or tentacles. Internally, they are missing circular muscles. The only muscles in their body wall extend from the mouth to the anus, so they are unable to creep along like earthworms. Instead, nematodes do a lot of wiggling to move from one place to another. One simple aspect of nematodes that may have helped them become so successful is their small size and permeable cuticle, which allows them to exchange gases and even some nutrients across their whole body surface.