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.
A pinworm. Adult pinworms live in the large intestine of humans.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.
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.
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