Nematodes are one of the most common phyla of animals, with over 20000 different described species. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts. Further, there are a great many parasitic forms, including pathogens in most plants and animals, humans included. Only the Arthropoda are more diverse.Roundworms are triploblastic protostomes. They are shaped like stereotypical worms, long and round in cross section, though without any segmentation. The body cavity is reduced to a narrow pseudocoelom, as is typical of very small animals. The mouth is anterior, and often surrounded by various flaps or projections used in feeding and sensation, with the anus slightly offset from the posterior. The epidermis secretes a layered cuticle that protects the body from drying out, from digestive juices, or from other harsh environments, as well as in some forms sporting projections that aid in locomotion.
Most free-living nematodes are microscopic, though a few parasitic forms can grow several metres in length. There are no circular muscles, so the body can only undulate from side to side. In order to actually get anywhere, the worm needs to be in contact with solid objects, its thrashing motions varying from mostly to completely ineffective at swimming. Different species feed on materials as varied as algae, fungi, small animals, fecal matter, dead organisms and living tissues.
Most free-living nematodes are microscopic, though a few parasitic forms can grow several metres in length. There are no circular muscles, so the body can only undulate from side to side. In order to actually get anywhere, the worm needs to be in contact with solid objects, its thrashing motions varying from mostly to completely ineffective at swimming. Different species feed on materials as varied as algae, fungi, small animals, fecal matter, dead organisms and living tissues.