

The nematodes or roundworms are one of the most common phyla of animals, with more than 20,000 described species. They inhabit freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants and animals, humans included. Only Phylum Arthropoda exceeds Nematoda in diversity. Nematodes are unsegmented, bilaterally symmetric, have triploblastic protostomes, and possess a complete digestive system. Nematodes are believed to be related to the arthropods and priapulids and grouped with them in the Ecdysozoa (the molting animals), the evolutionary is unresolved. Because most living forms are microscopic, the discovery of their ancient ancestors as fossils is unlikely. However, one species of extant parasitic nematode can reach 13 meters in length. lacking notable mineralized body parts, chances for fossilization of soft tissues is rare, and require special circumstances as existed for the specimens from the Cambrian Chengjiang fossils shown below, which also suggest their evolutionary appearance in the Precambrian. Nematode fossils have been found in amber.