What are annuli? What is their function.

Explanation

**Step-by-Step Solution:**

1. **Definition of Annuli**: Annuli are ring-like structures found in organisms belonging to the phylum Annelida, which includes segmented worms such as earthworms and leeches.

2. **Location of Annuli**: These structures are located on the external surface of the body of annelids. They appear as grooves or rings that encircle the body.

3. **Metameric Segmentation**: Annelids exhibit a characteristic known as metameric segmentation, where their bodies are divided into repeated segments called metameres. The presence of annuli contributes to this segmentation.

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