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Worms (Helminthes).
An Introduction to Worms.
Worm Galleries Directory.

An oligochaete worm with ingested Trachelomonas.
An oligochaete worm with ingested Trachelomonas.

  Worms: An Overview.

Worms: Introduction Annelids Nematodes Flatworms Parasitic

In earlier times, the term Worm was loosely used to describe any small animal having a long slender body without appendages. It was used to include not only worms but caterpillars and other insect larvae, and even creatures as unrelated as rotifers and centipedes, all lumped together in the now obsolete category Vermes.

In modern classifications, worms are still recognized as a highly diverse group, and there are a number of systems in use for classifying them. Depending on the particular system, there are around ten worm phyla. Most worms are marine, and many live their lives in tubes which they construct in the sand of the ocean floor. Others live as internal parasites of marine animals including whales and fish, molluscs and octopus.

Many of the parasitic worms of terrestrial animals have larval stages in fresh water, and these can be encountered in water samples collected from lakes and ponds. The three worm phyla of most interest to freshwater biologists are presented below. Additionally, the category "Parasitic" is included, which acknowledges the immense impact of worm parasites upon human and other animal communities.

  1. Annelida (segmented worms).
    Most annelid worms are macroscopic and free-living, with elongated, contractile bodies divided into a number of segments, most of which bear chaetae (or setae), or bristles. Some have a sucker at one or both ends.

    Annelid worms are divided into three classes:

    • Oligochaete Worms.
      The term oligochaete means "not many bristles". The group includes terrestrial forms such as the earthworm and many microscopic forms inhabiting fresh water.

    • Polychaete Worms.
      The term means "many bristles". These are the extremely numerous marine bristleworms. Very few are found in freshwater.
      None are shown in these galleries.

    • Hirudinae (Leeches).
      Leeches are found in fresh water. There are around 500 species.

  2. Nematoda (nematodes, roundworms).
    Probably the most abundant animals on Earth. They are unsegmented, and are sometimes called roundworms. Many are parasitic, and in whales can grow to 9 metres or so, but the free-living forms found in fresh water are usually less than a millimetre.

  3. Platyhelminthes (flatworms).
    This group contains all the bilaterally symmetrical worms and includes ribbon and leaf-shaped forms. They are the amongst the simplest members of the bilaterally symmetrical metazoa.

    Flatworms are divided into three classes:

    • Turbellarian Worms.
      This class contains all the free-living flatworms. There are also some commensal and parasitic species. The mouth leads to a blind-ended gut which may be simple or branched. There is no anus. Most are marine, but the freshwater forms include the well-known planarians.

    • Cestoda.
      Includes all the tapeworms. All forms are parasitic.

    • Trematoda.
      All flukes, including the liver fluke. All are parasitic.

  4. Parasitic Worms.
    Not a taxonomic group, as there are parasitic forms in many worm phyla, but is included here for the convenience of those primarily interested in the parasitic forms.
    It contains no specimens which have not been covered under the taxonomic groupings.

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