Nematoda: Roundworms

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Nematoda: Roundworms

Nematode
Light micrograph of a nematode or roudworm. FRANK FOX/Science Photo Library/Getty Images

Nematoda is the phylum of the Kingdom Animalia that includes roundworms. Nematodes can be found in almost any type of environment and include both free-living and parasitic species. Free-living species inhabit marine and freshwater environments, as well as the soils and sediments of all of the various types of land biomes. Parasitic roundworms live off of their host and can cause disease in the various types of plants and animals they infect. Nematodes appear as long, thin worms and include pinworms, hookworms, and trichinella. They are among the most numerous and diverse organisms on the planet.

Nematoda: Types of Nematodes

Nematodes are broadly categorized into two main groups: free-living and parasitic. Free-living nematodes feed on organisms in their environment. Parasitic types feed off of a host and some also live within the host. The majority of nematodes are non-parasitic. Nematodes vary in size from microscopic to reaching lengths of over 3 feet. Most nematodes are microscopic and often go unnoticed.

Free-living Nematodes

Free-living nematodes reside in both aquatic and terrestrial habitats. Soil nematodes play a vital role in agriculture and the recycling of nutrients and minerals in the environment. These organisms are commonly grouped into four main types based on their feeding habits. Bacteria-eaters feed exclusively on bacteria. They help to recycle nitrogen in the environment by decomposing bacteria and releasing excess nitrogen as ammonia. Fungi-eaters feed on fungi. They have specialized mouth parts that enable them to pierce the fungal cell wall and feed on the internal fungal parts. These nematodes also aid in decomposition and the recycling of nutrients in the environment. Predatory nematodes feed off of other nematodes and protists, such as algae, in their environment. Nematodes that are omnivores feed on different types of food sources. They may consume bacteria, fungi, algae, or other nematodes.

Parasitic Nematodes

Parasitic nematodes infect various types of organisms including plants, insects, animals, and humans. Plant parasitic nematodes typically live in soil and feed on cells in plant roots. These nematodes live either externally or internally to the roots. Herbivore nematodes are found in the orders Rhabditida, Dorylaimida, and Triplonchida. Infection by plant nematodes damages the plant and causes a reduction in water uptake, leaf expansion, and the rate of photosynthesis. Damage to plant tissues caused by parasitic nematodes can leave the plant vulnerable to disease causing organisms such as plant viruses. Plant parasites also cause diseases such as root rot, cysts, and lesions that reduce crop production.

Parasitic nematodes that infect humans include Ancylostoma duodenale and Necator americanus - hookworm; Enterobius vermicularis - pinworm; Strongyloides stercoralis - threadworm; Trichuris trichiura - whipworm; and Trichinella spiralis - trichina worm. These parasites infect the gastrointestinal tract through the consumption of contaminated food or water. Some nematodes may also be transmitted to humans by pets or insect vectors such as mosquitoes or flies.

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Nematoda Anatomy

Nematode Micrograph
Aquatic (fresh water) nematode living in pond water among cyanobacteria. NNehring/E+/Getty Images

Nematoda Anatomy

Nematodes are unsegmented worms with long, thin bodies that narrow at both ends. Major anatomical characteristics include bilateral symmetry, a cuticle, a pseudocoelom, and a tubular excretory system.

  • Cuticle: A protective outer layer that is composed mainly of collagens that are cross-linked. This flexible layer acts as an exoskeleton that helps to maintain body shape and enables movement. Molting of the cuticle at different stages of development allows nematodes to increase in size.
  • Hypodermis: The hypodermis is an epidermis composed of a thin layer of cells. It lies directly below the cuticle and is responsible for secreting the cuticle. The hypodermis thickens and bulges into the body cavity at certain places forming what are known as hypodermal cords. Hypodermal cords extend along the length of the body and form the dorsal, ventral, and lateral chords.
  • Muscles: A layer of muscles lies beneath the hypodermis layer and runs longitudinally along the internal body wall.
  • Pseudocoelom: A pseudocoelom is a body cavity filled with fluid that separates the body wall from the digestive tract. The pseudocoelom acts as a hydrostatic skeleton, which helps to resist external pressure, aids in locomotion, and transports gases and nutrients to body tissues.
  • Nervous System: The nematode nervous system contains a nerve ring near the mouth region that is connected to longitudinal nerve trunks that run the length of the body. These nerve trunks connect the anterior nerve ring (near the mouth) to the posterior nerve ring (near the anus). In addition, dorsal, ventral, and lateral nerve chords connect to sensory structures through peripheral nerve extensions. These nerve chords aid in movement coordination and the transmission of sensory information.
  • Digestive System: Nematodes have a three-part tubular digestive system consisting of a mouth, intestine, and anus. Nematodes have lips, some have teeth, and some may have specialized structures (ex. stylet) that help them to obtain food. After entering the mouth, food enters the muscular pharynx (esophagus) and is forced to the intestine. The intestine absorbs nutrients and excretes waste products. Undigested material and waste is moved along to the rectum where it is passed through the anus.
  • Circulatory System: Nematodes do not have an independent circulatory system or cardiovascular system as do humans. Gases and nutrients are exchanged with the external environment through diffusion across the surface of the animals body.
  • Excretory System: Nematodes have a specialized system of gland cells and ducts that excrete excess nitrogen and other waste through an excretory pore.
  • Reproductive System: Nematodes reproduce primarily through sexual reproduction. Males are typically larger than females as the females must carry large numbers of eggs. Reproductive structures in females include two ovaries, two uteri, a single vagina, and a genital pore that is separate from the anus. Reproductive structures in males include testes, a seminal vesicle, vas deferens, and a cloaca. The cloaca is a cavity that serves as a common channel for both sperm and excrement. During copulation, males use slender reproductive body parts called spicules to open the female genital pore and aid in the transfer of sperm. Nematode sperm lack flagella and migrate toward female eggs using amoeba-like movement. Some nematodes can reproduce asexually by parthenogenesis. Others are hermaphrodites and have both male and female reproductive organs.

Sources:

  • "Nematoda." Animal Sciences. . Retrieved January 10, 2017 from Encyclopedia.com: http://www.encyclopedia.com/science/news-wires-white-papers-and-books/nematoda
  • "Soil Nematodes" Online primer: Soil Biology Primer. . Retrieved January 10, 2017 from NRCS.USDA.gov: https://www.nrcs.usda.gov/wps/portal/nrcs/detailfull/soils/health/biology/