What Are the Parts of a Roundworm?
Roundworms, in the phylum Nematoda, number well over 20,000 species. Several differences between them and flatworms exist. Interestingly, the number of cells in an individual nematode is species-specific and varies between species. The anatomy of a roundworm is well documented, and species can be parasitic or free-living. To be categorized as a roundworm, a species must contain several particular body parts.
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Cuticle
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Roundworms have an unsegmented exterior that is unusual in that it is not a collection of cells like human skin. The cuticle that covers roundworms is a collection of cellular secretions; it is similar to the covering of arthropods, such as insects and crustaceans, though, more flexible. The proteinaceous and lipid composition of the cuticle protects the parasitic species from the digestive enzymes of their hosts.
Muscles
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While the roundworms are radially symmetrical, their muscles do not form rings. Longitudinal muscles run from one end to the other, allowing for movement through "dorso-ventral undulation," according to the University of California at Davis. Like the bone and muscle action in humans, roundworms use hydrostatic pressure to act against the contractions of their muscles.
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Neural Ring
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Roundworms have a neural ring connected to nerve cords along the back and belly and sensory neurons in the head. These are also connected to motor neurons that cause muscular contraction, though, unlike most animals, the muscles branch into the neurons rather than the other way around.
Digestive System
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Roundworms are the lowest form of life to develop a "tube within a tube"-style digestive system, not unlike our own although theirs is much more rudimentary. A distinguishing characteristic of nematodes is the beginning of their alimentary canal. Triradiate mouthparts, or lips, open onto a pharynx, where food is crushed. As food passes through the linear alimentary canal, nutrients are extracted, and waste is excreted through the animal's anus -- where the digestive tract terminates.
Respiratory System
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Roundworms lack a respiratory or circulatory system. Gas exchange takes place through diffusion across the cuticle layer, and osmoregulation -- the balance between salt and water performed by the kidneys in humans -- occurs through the excretory canals running along the worm's sides.
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References
- University of California Museum of Paleontology: Introduction to the Nematoda
- University of California at Davis: Introduction to Nematodes
- California State University Bakersfield: Nematoda Anatomy
- Lander University: Cephalobus
- North Carolina State University Plant Nematode Genomics Group: Lecture 3 Nematode Anatomy (PDF)
Resources
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