Nematoda

The Phylum Nematoda consists of the species commonly known as roundworms. There are approximately 12,000 described species, but the actual number could be many times higher. Nematode worms are extremely abundant; often, several hundred species, and as many as a million individuals, inhabit a square yard of soil. Nematodes are also extremely varied ecologically. They are found in almost every imaginable habitat , including terrestrial (land-based), freshwater, and saltwater ecosystems , as well as within other organisms as parasites. Nematodes can be herbivorous, carnivorous, or parasitic, and include both generalists (who make use of a wide variety of resources) and specialists (who make use of only particular resources). They play a particularly critical role in decomposition and nutrient cycling, where they are often the intermediate decomposers that partly break down organic materials so that they can then be dealt with by bacterial decomposers.

Characteristics of Nematodes

Roundworms are small, slender, unsegmented worms which are tapered at both ends. They have a circular cross section. Different species of nematodes are often difficult to distinguish because of their fairly uniform external morphology, or outer appearance.

Nematodes are characterized by an external (outer) layer of cuticle that is secreted by the hypodermis underneath it. The cuticle is somewhat rigid. However, it is flexible enough to permit bending and stretching, and can be penetrated by gases and water. The cuticle is molted , or shed, several times during the worm's growth. The hypodermis underlying the cuticle is a syncitium—that is, it consists of large cells with more than one nucleus. A layer of muscle cells is found beneath the hypodermis. All nematode muscle fibers run lengthwise along the animal's body. This single, unvaried orientation limits nematodes to their characteristic, and somewhat awkward, pattern of movement, a flailing whiplike motion that is produced by alternate contractions (shortenings and thickenings) of muscle cells on either side of the animal's body. The rigidity of the cuticle layer also limits the motion of nematodes.

Nematodes lack a true coelom (body cavity) since their internal cavity is not lined by cells originating from the embryonic mesoderm. Instead, they possess a fluid-filled pseudocoel (incomplete coelum) that contains the intestine and reproductive organs.

The nematode nervous system is characterized by an rear nerve ring around the area of the pharynx (area deep inside the mouth cavity) and two pairs of lengthwise nerve cords that run down the body. There are also dorsal (back) and ventral (belly) nerve cords as well as a set of lateral nerve cords across the body. These nerve cords transmit sensory information and coordinate movement. Nematodes have a variety of sensory receptors, including tactile (touch) receptors at the front and back ends of the body, and chemosensory (chemical-sensitive) cells at the front end. They also have light-sensitive organs organized either in ocelli (simple eyes) or distributed along the surface of the body.

Nematodes have a complete gut with a mouth and an anus. Teeth, which are used to pierce animal or plant matter, aid in obtaining food. The pharynx is muscular and pumps food through the gut, and nutrients are absorbed in the intestine. There is no internal system of circulation, so the transport of nutrients and wastes is achieved by diffusion (scattering). Specialized cells for excretion, which are known as rennette cells and are unique to the phylum, remove nitrogen-laden wastes. These are expelled from the nematode directly through the body wall, in the form of ammonia.

Nematodes breathe across their entire body surface. This gas exchange strategy is adequate because of the small size of the worms, which means they have a high ratio of surface area to volume.

The majority of nematodes are dioecious ; that is, the sexes are separate. Some species, however, are hermaphroditic, having both male and female reproductive organs. In dioecious species, males have a specialized spine for sexual reproduction that is used to open the female's reproductive tract and to inject sperm. Nematode sperm is unusual in that the sperm cells do not have flagella, and move using an amoeboid motion (crawling). While some species are live-bearing, most lay eggs. Eggs escape through a midbody hole called the gonopore in the female. There is no distinct larval stage. Eggs develop directly into juveniles that generally resemble the adults except that they lack mature reproductive organs. Nematodes are also characterized by an unusual feature called "eutely," in which every individual of a given species has exactly the same number of cells. This cell number is achieved by the end of the developmental period, so that subsequent growth of the animal involves increases in cell size rather than in cell number.

Nematodes of Particular Interest

Some well-known nematode parasites include hookworms, pinworms, and heartworms. Also included are Trichinella spiralis, which is responsible for trichinosis and uses both pigs and humans as hosts, and filarial worms, which are the primarily tropical parasites responsible for the diseases elephantiasis and river blindness.

The nematode Caenorhabditis elegans is one of the most well-studied living species and has served as a biological model organism for genetic and developmental studies. It was the first multicellular organism for which a complete DNA sequence was obtained.

Jennifer Yeh

Bibliography

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.

Gould, James L., and William T. Keeton. Biological Science, 6th ed. New York: W. W. Norton and Co., 1996.

Hickman, Cleveland P., Larry S. Roberts, and Allan Larson. Animal Diversity.

Dubuque, IA: Wm. C. Brown, 1994.

Nematode

Nematodes, also called roundworms, are members of the animal phylum Nematoda. These worms have a complete digestive system and are more complex than the flatworms (phylum Platyhelminthes) but lack a circulatory system and other advanced features found in the annelids (segmented worms). The Nematoda is one of the largest animal phyla, with over 15,000 described species. Many more species remain to be discovered because most nematodes are microscopic in size and not easily observed.

Nematodes are an extremely diverse group and are common in most habitats. These aquatic worms are abundant in freshwater and marine ecosystems but also inhabit the moisture film around soil particles. A small handful of soil may contain several thousand individuals. Nematodes even occur in desert soils and in Antarctica.

Many kinds of nematodes are parasites , inhabiting vertebrates (including humans) or invertebrates. Others are parasites of plants and feed on or live within roots, tubers, bulbs, and other below-ground plant parts. A few unusual species live inside leaves, stems, or seeds. Some of the nonparasitic, free-living nematodes are predators of other minute organisms. Most free-living nematodes feed on bacteria or fungi. Their activities are important in the decomposition of organic matter and recycling of nutrients.

see also Animalia; Parasitic Diseases; Platyhelminthes; Symbiosis

Robert McSorley

Bibliography

Poinar, George O., Jr. The Natural History of Nematodes. Englewood Cliffs, NJ: Prentice Hall, 1983.

Ruppert, Edward E., and Robert D. Barnes. Invertebrate Zoology, 6th ed. Philadelphia, PA: Saunders College Publishing, 1994.

Nematoda (eelworms, roundworms, threadworms; phylum Aschelminthes) A class of worms (a phylum in some classifications) which vary greatly in size up to about 5 cm, although most are microscopically small. The whole body is covered by a tough cuticle that has flanges, and may also have ridges or spines. Nematodes are spindle-shaped (the nearly perfect cylindrical shape is a diagnostic feature), have bilateral symmetry, with strikingly radial or biradial arrangements of structures around the mouth (also a diagnostic feature), and are unsegmented. They lack true cilia, have a pseudocoelom, and the head is not distinct from the rest of the body. There is a very large number of species, morphologically all similar, occurring as parasites in plants and animals, and as free-living forms. They are first known from rocks of Carboniferous age.

Nematoda A phylum of pseudocoelomate invertebrates comprising the roundworms. They are characterized by a smooth narrow cylindrical unsegmented body tapered at both ends. They shed their tough outer cuticle four times during life to allow growth. The microscopic free-living forms are found in all parts of the world, where they play an important role in the destruction and recycling of organic matter. The many parasitic nematodes are much larger; they include the filaria (Wuchereria) and Guinea worm (Dracunculus), which cause serious diseases in humans.

Nematoda (nematodes, eelworms, threadworms, roundworms; phylum Aschelminthes) Class of aschelminth worms (a phylum in some classifications) which vary greatly in size, from about 1 mm to 5 cm. The cuticle has flanges, and may also have ridges or spines. Morphologically they are all similar, but they occur both as parasites in plants and animals, and as free-living forms. They are first known from rocks of Carboniferous age (e.g. Scorpiophagus).