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Annelids: Types & Structure

Instructor: Lisa Roundy

Lisa has taught at all levels from kindergarten to college and has a master's degree in human relations.

Annelids are worms with segmented bodies. Annelids are divided into three main classes. A brief description and example of each class will be provided in this lesson.

Annelids

As a child, you may have enjoyed squishing your toes in the mud left behind by a heavy rain. If you did, there is a good chance you may have also noticed something besides your toes wiggling in the mud. This is because you were joined by earthworms that had been drawn to the surface after the rain.

Segmented worms, like the earthworms in our example, make up the Phylum Annelida. These worms, referred to as annelids, do not have legs or a hard skeleton. They also have a developed digestive system, a respiratory system, and a circulatory system. Some annelids are parasitic and others obtain their own food from the environment. Many known species of annelids belong in one of three classes: Polychaeta, Oligochaeta, and Hirudinea.

Polychaeta

Example of the class Polychaeta
polychaete

Annelids that belong to the class of Polychaeta account for the largest number of these worms. Sandworms are an example of polychaetes. Polychaetes are typically marine annelids that have many bristles protruding from their bodies. Because of this, they are also commonly known as bristle worms. Polychaetes reproduce sexually and can be found burrowing on the shore, floating near the surface of the water, or living in the depths of the ocean. Unlike the other classes of annelids, which all have the same general appearance, polychaetes have a wide variety of body forms.

Oligochaeta

Example of the class Oligochaeta.
Oligochaeta

Annelids that belong to the class of Oligochaeta do not have many bristles protruding from their body like the polychaetes. In fact, any protrusions that may exist unnoticeable on a oligochaete without a careful examination. This type of annelid includes fewer species, but consists of a larger number of individuals than any other class of annelid. Oligochaetes are typically found living in soil or fresh water and most feed on decaying matter. Every oligochaete is hermaphroditic, or possesses reproductive organs of both sexes; however, oligochaetes rarely self-reproduce. Earthworms are a well-known, and commonly seen, example of oligochaetes.

Hirudinea

Example of the class Hirudinea.
leech

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