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Roundworms - Advanced
Douglas Wilkin, Ph.D.
Jennifer Blanchette
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Printed: December 22, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jennifer Blanchette
www.ck12.org
C HAPTER
Chapter 1. Roundworms - Advanced
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Roundworms - Advanced
• Describe the structural features of roundworms that distinguish them from flatworms.
When most people picture a worm, do they picture a roundworm?
Actually, they do not. Whereas flatworms are flat, roundworms obviously appear round. With over 80,000 species,
there are plenty of different types of roundworms. But these are still not the earthworms most people picture when
they think of worms.
Characteristics of Roundworms (Nematoda)
Roundworms make up the phylum Nematoda. This is one of the most abundant animal phyla, with greater than
80,000 known species (although not all of them have been classified). They range in size from less than one
millimeter to 7 meters. Roundworms are found nearly everywhere, including aquatic and terrestrial environments
as well as parasitically on or within a variety of plants and animals. In the next several sections we will review the
anatomy of roundworms, highlighting the features specific to this phylum.
Anatomy of Roundworms
As the name implies, roundworms are cylindrical in shape. Like flatworms, roundworms have excretory, nervous,
and reproductive systems. Another similarity to flatworms is that they lack an enclosed circulatory and respiratory
system. The two most prominent features that distinguish roundworms from flatworms are the presence of a
pseudocoelom, or partially developed body cavity, and a complete digestive tract with two openings, a mouth and an
anus. A pseudocoelom differs from a true coelom in that the cavity is lined with mesoderm only on the ectodermal
side. The presence of a body cavity allows circulatory fluid to flow freely throughout the body of the organism and
facilitates the exchange of material between cells (including the release of waste). A complete digestive tract with
separate openings for the mouth and anus allows the animals to simultaneously feed, digest, and eliminate waste.
The main differences between flatworms and roundworms are summarized in the Comparison Table.
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TABLE 1.1: A Comparison of Flatworms and Roundworms
Flatworms (Platyhelminthes)
Flat body
Primitive gut (1 opening)
Acoelomate (no body cavity)
Roundworms (Nematoda)
Round body
Complete digestive tract (2 openings)
Pseudocoelem (partial body cavity)
Roundworms have a thick substance called a cuticle on the surface of their bodies that is secreted by the outer
epidermal cells. The cuticle is fairly rigid and limits the volume of the worm. This allows the build up of hydrostatic
pressure from fluid accumulated in the worm and contributes to what is called a hydrostatic skeleton. The force
of the hydrostatic pressure allows the worm to maintain its cylindrical shape. This hydrostatic force along with
contractions from the muscles lining the pseudocoelom allows the worm to move along solid surfaces. As is the case
for flatworms, there are both free-living and parasitic species of roundworms. The nervous system of nematodes,
like that of flatworms, is simple and has a concentrated nerve center in the anterior, or head, region. Nematodes also
have sensory organs connected to their nervous system. They generally have either one, termed the amphid, or two:
an anterior amphid and a posterior phasmid. An example of a roundworm, the soybean cyst nematode, is shown in
the Figure 1.1.
FIGURE 1.1
Scanning electron micrograph (microscopic photograph) of a soybean cyst nematode magnified 1000x.
Vocabulary
• amphid: A circular depression situated laterally at the anterior end of nematodes and believed to be chemoreceptors.
• coelom: A fluid filled cavity formed within the mesoderm; it forms between the digestive cavity and the body
wall.
• cuticle: A thick organic layer surrounding the outer surface of nematodes and arthropods; a waxy waterproof
covering over the aerial surfaces of a plant.
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Chapter 1. Roundworms - Advanced
• hydrostatic skeleton: A structure consisting of a fluid-filled cavity surrounded by muscles; it is used to change
an organism’s shape and produce movement.
• phasmid: One of a pair of circular depressions situated laterally at the posterior end of nematodes and believed
to be chemoreceptors.
• pseudocoelom: A partially developed, fluid-filled body cavity that is lined with mesoderm only on the
ectodermal side.
Summary
• The two most prominent features that distinguish roundworms from flatworms are the presence of a pseudocoelom, or partially developed body cavity, and a complete digestive tract with two openings, a mouth and an
anus.
• Roundworms have a thick substance called a cuticle on the surface of their body that is secreted by the outer
epidermal cells.
• There are both free-living and parasitic species of roundworms.
Practice
Use this resource to answer the questions that follow.
• Introduction to the Nematoda at http://www.ucmp.berkeley.edu/phyla/ecdysozoa/nematoda.html .
1.
2.
3.
4.
5.
6.
How prevalent are species of Nematoda?
What groups are nematodes most closely related to?
What is unique about a nematodes nerve and muscle cells?
How are nutrients distributed within a nematode?
What is cryptobiosis?
Where can you find nematodes?
Review
1.
2.
3.
4.
5.
What are the main differences between roundworms and flatworms?
What differentiates a pseudocoelom from a true coelom?
Why is a body cavity so important as an evolutionary development?
How is the roundworm’s digestive system superior to the digestive system of flatworms?
What allows roundworms to maintain their shape and move?
References
1. United States Department of Agriculture. http://commons.wikimedia.org/wiki/Image:Soybean_cyst_nematod
e_and_egg_SEM.jpg . Public Domain
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