Phylum Annelida: Characteristics, Classes & Examples

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  • 0:05 Who belongs in Phylum…
  • 0:28 Characteristics of Annelids
  • 2:52 Classes and Examples…
  • 5:21 Lesson Summary
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Lesson Transcript
Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

Phylum Annelida contains a variety of worms and their relatives. They are found everywhere from marine and freshwater habitats to damp soil. Learn more about annelids here and take a short quiz.

Who Belongs in Phylum Annelida?

If you are an avid gardener, you may often encounter a common member of the phylum Annelida. The earthworm, well-liked for creating healthy soil for our earth, is perhaps the most recognizable creature in this group. Known for their long, segmented bodies, annelids, such as earthworms, leeches, and many marine worms, certainly have their place in the world. In this lesson, we will learn more about phylum Annelida and gain an understanding of what makes them unique.

The earthworm is most familiar of the annelids

Characteristics of Annelids

Annelids are found in many different areas and environments. We dig earthworms up in damp soil, and may find leeches attached to our body after swimming in a lake. Although earthworms are the most familiar, over two-thirds of all annelids live in marine habitats. Annelids range in size from under a millimeter to an astounding 3 meters in length (The Australian earthworm). Some marine species have brilliant colors and unique shapes.

When you picture the body of an annelid, you probably don't imagine a very complex structure. An earthworm, for example, appears to essentially be a thin tube of skin filled with a gooey dirt-like substance. However, this appearance is deceiving. Annelids have many of the same bodily structures and organs as humans. Let's take a closer look at their general characteristics.

As far as worm-like creatures go, the annelids are the most complex. All members of Annelida have segmented bodies, a trait that is visibly evident on an earthworm. They are cylindrical in shape, though some are flattened, exhibiting bilateral symmetry. Although there is no actual head, in the front of the body we find a brain that connects to a nerve cord that runs the length of the body. Each body segment also has a cluster of nerve cells coming off this main nerve cord. In this front area, we also find a mouth and sensory organs.

It may come as a surprise that annelids also have a circulatory system for moving blood, nutrients, and oxygen. This system has two main blood vessels running the length of the body, and - you guessed it - smaller segmented vessels in each body segment. Some of these vessels are enlarged and act as heart-like pumps. In addition, a continuous digestive tract also runs down the entire body, complete with an esophagus, intestines, and segmented excretory organs to remove liquid waste from the body.

The simple body plan of an annelid is known as a tube-within-a-tube, which is exactly what it sounds like. There is a long tube-shaped compartment on the inside of the body that consists mainly of the digestive tract. This is enclosed by an outer tube, with a space between the two called the coelom. This space is filled with fluid, thus suspending the inner compartment. Because of this fluid, annelids have a firmness similar to that of water balloons. In addition, muscles in the outer tube give the annelid the ability to move, with an epidermis serving as a protective covering.

Classes and Examples of Annelids

There are three classes within phylum Annelida. First, we will take a look at Oligochaeta, the class containing the ever-popular earthworm (as well as many aquatic worms). Earthworms are often used in dissection labs to observe and analyze body parts of animals in this phylum. As previously mentioned, the segments are prominent on earthworms, giving them a ringed appearance. Each segment contains tiny bristles called setae, which help them grip the ground as they burrow.

The red wiggler, a member of Oligochaeta
red wiggler

Earthworms have over one hundred segments. On their first segment we find a mouth, which is used to essentially suck in soil. When earthworms take in soil, they grind up the particles with organic matter, mixing it well. The soil matter travels the length of their body, and is then finally expelled out the anus on the last segment. This process is crucial for the health of soil, as it is mixed and aerated as the worm travels.

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