The roundworms (Phylum Nematoda) are one of the most common phyla of animals, with over 20,000 different described species. They are ubiquitous in 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 the Arthropoda are more diverse.
The roundworms were originally named the Nemata by Nathan Cobb in 1919. Later they were demoted to a class Nematoda in the Aschelminthes, and then restored to phylum Nematoda.
Morphology
Roundworms are triploblastic protostomes with a complete digestive system. They are thin and are round in cross section, though they are actually bilaterally symmetrical. The body cavity is reduced to a narrow pseudocoelom. The mouth is often surrounded by various flaps or projections used in feeding and sensation. The portion of the body past the anus or cloaca is called the "tail." The epidermis secretes a layered cuticle made of keratin 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. This cuticle is shed as the animal grows.
Most free-living nematodes are microscopic, though a few parasitic forms can grow to several metres in length. There are no circular muscles, so the body can only undulate from side to side. Contact with solid objects is necessary for locomotion; its thrashing motions vary from mostly to completely ineffective at swimming.
Roundworms generally eat bacteria, algae, fungi and protozoans, although some are filter feeders. Excretion is through a separate excretory pore.
Reproduction is usually sexual. Males are usually smaller than females (often very much smaller) and often have a characteristically bent tail for holding the female for copulation. During copulation, one or more chitinized spicules move out of the cloaca and are inserted into genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm.
Eggs may be embryonated or unembryonated when passed by the female, meaning that their fertilized eggs may not yet be developed. In free-living roundworms, the eggs hatch into larva, which eventually grow into adults; in parasitic roundworms, the life cycle is often much more complicated.
Roundworms have a simple nervous system, with a main nerve cord running along the ventral side. Sensory structures at the anterior end are called amphids, while sensory structures at the posterior end are called phasmids.
Free-living species
In free-living species, development usually consists of four molts of the cuticle during growth. Different species feed on materials as varied as algae, fungi, small animals, fecal matter, dead organisms and living tissues. Free-living marine nematodes are important and abundant members of the meiobenthos. One roundworm of note is Caenorhabditis elegans, which lives in the soil and has found much use as a model organism.
Parasitic species
Parasitic forms often have quite complicated life cycles, moving between several different hosts or locations in the host's body. Infection occurs variously by eating uncooked meat with larvae in it, by entrance into unprotected cuts or directly through the skin, by transfer via blood-sucking insects, and so forth.
Important parasites on humans include whipworms, hookworms, pinworms, ascarids, and filarids. The species Trichinella spiralis, commonly known as the trichina worm, occurs in rats, pigs, and humans, and is responsible for the disease trichinosis. Baylisascaris usually infests wild animals but can be deadly to humans as well. Haemonchus contortus is one of the most abundant infectious agents in sheep around the world, causing great economic damage to sheep farmers.
Phylogeny
The common presence of a pseudocoelom is no longer considered evidence that the pseudocoelomate phyla are all related, but a few groups are still probably close relatives of the Nematoda. Of special note here are the Nematomorpha, or horse-hair worms, which have larvae parasitic in arthropods and free-living adults. The Arthropods have also been considered to be possible relatives of these groups, the common process of ecdysis (molting) being evidence for this. Together, the molting animals form the clade Ecdysozoa.
