Metamerism in Annelids

Lesson Transcript
Instructor: Wendy McDougal

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

Annelids are segmented worms belonging to the phylum Annelida, and their segmented nature serves as an example of metamerism. Learn the name of their segments, the division between the segments, their internal septum, and the purpose of metamerism. Updated: 11/30/2021

Definition of Metamerism

If you were asked to describe an earthworm, what would you say? You might explain that it is small, thin and cylindrical in shape. It is pinkish in color, and is usually found burrowing through the soil. And you would most likely point out that its body seems to be divided into tiny vertical sections. The segmented body of an earthworm is its trademark, and truly sets it apart from many other types of worms. In the science world, body segmentation such as this is known as metamerism.

Metamerism means repeating segments that make up a whole. Although this body type is found in several different groups in the animal world, in this lesson we'll look at this trait in members of the phylum Annelida. In particular, we'll focus on an annelid with very pronounced segments, the earthworm. The question becomes, are these segments just for looks, or do they serve a purpose? In this lesson, learn about metamerism in annelids and understand what function they serve.

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  • 0:01 Definition of Metamerism
  • 0:58 A Body Made of Stacked Rings
  • 2:56 The Purpose of Metamerism
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A Body Made of Stacked Rings

If you have a lump of clay around, you can easily replicate the body of an earthworm. Just take a ball of the soft clay, and roll it on the table until it has a wormy shape. Then, take a toothpick to draw the segment lines on the outside of the body, and voila, you have a segmented earthworm. Or do you?

Upon initial examination, it may look as though the segments on an earthworm are like those on the clay version. They appear to just be vertical lines on the outside of the body, like the ribbing on a pair of leggings. However, segmentation on an actual annelid actually goes much deeper than you might think. In fact, it would be much more accurate to say that an annelid is composed of separate rings that are stacked together, like a stack of candy Life Savers. This would explain the literal meaning of annelid, which is 'little rings.'

Let's take a closer look at the anatomical features of our segmented earthworm. The lines we see externally separating each segment are called annuli. Now imagine carefully slicing along each line to divide the earthworm into its segments. Internally, we would find repetition of tissues and organs, which demonstrates the concept of metamerism. Not only is the earthworm divided on the outside, but it's split internally as well.

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