Phylum Nematoda: Classes, Characteristics & Examples

Phylum Nematoda: Classes, Characteristics & Examples
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  • 0:01 Phylum Nematoda: The…
  • 1:54 Characteristics of…
  • 3:08 Classes of Phylum Nematoda
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Lesson Transcript
Instructor: Joanne Abramson

Joanne has taught middle school and high school science for more than ten years and has a master's degree in education.

With 25,000 species discovered and an estimated 900,000 waiting to be discovered, phylum Nematoda is one the most species-rich phyla in the animal kingdom. This lesson discusses this and many other interesting facts about these tiny worms.

Phylum Nematoda: The Roundworms

If you have ever owned a dog, chances are pretty good that you have had personal contact with phylum Nematoda. Roundworms, hookworms, and heartworms are all members of phylum Nematoda, and they are all common canine maladies. Most dogs will become infected with at least one species of nematode at some point in their life. In fact, since roundworms are so common, and since they also pose a risk to humans, the Centers for Disease Control recommend having your dog regularly de-wormed.

Phylum Nematoda consists of the nematodes, or roundworms. Both terms are apt descriptions. Visually, the members of this phylum are described as being long, thin, and hair-like. This is how they received their scientific name. Nematode is from the Greek roots nemat-, meaning thread, and -odes, meaning like or resembling. They have also been described as similar to a long, thin, round tube…hence, roundworms. Most species are microscopic, however a few are over a meter long. The largest known nematode infects sperm whales and can grow up to 9 meters. That's almost 30 feet long! The root-knot nematode, Meloidogyne incognita, is a parasite that infects plants.

Phylum Nematoda is one of the most prolific phyla in the animal kingdom. Although there are only slightly more than 25,000 known species, scientists estimate there are probably another 900,000 species waiting to be discovered. They have been found in nearly every possible environment, including as parasites in a wide variety of plants and animals. They don't, however, like dry locales. One species of nematode, the sour paste nematode, has been found in book binding paste and German beer mats!

Characteristics of Phylum Nematoda

Nematodes are characterized by their long tube-like appearance that lacks a visually distinct head or tail. In other words, they have a head and a tail, it's just really hard to tell which end is which. A second unique feature of this phyla is the cuticle, secreted by a layer of skin cells. The cuticle is the strong but flexible outer layer that serves to support and protect the worm, aid with movement, and help maintain the internal pressure that gives the worm its characteristic round shape.

They have a very simple body plan. Below the skin cells is a layer of muscle cells. The muscle cells are controlled by two nerves that run the length of the roundworm's body. One down the dorsal (back) side, and one down the ventral (belly) side. Roundworms have a complete digestive tract with a mouth at the anterior (head) end and an anus at the posterior (tail) end.

Nematodes are a very fertile bunch, with as many as 90,000 individuals found on a single rotting apple. Knowing this, it should not be much of a surprise that most of the internal space of the roundworm is taken up by sex organs, either oviducts or testes. They do not possess a circulatory or respiratory system.

Classes of Phylum Nematoda

Phylum Nematoda is divided into classes. Two of these are class Enoplea and class Chromadorea, each having at least one sub-class. Both classes consist of free-living and parasitic roundworms that live in freshwater, marine water, the soil, or a host. The classes are distinguished from each other through anatomical features such as a smooth or segmented cuticle and the shape of the digestive tract.

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