What is the most prominent feature of the phylum Annelida?
What type of body cavity do these animals have?

Explanation

### Step-by-Step Solution:

1. **Identify the Phylum**: The question asks about the phylum Annelida, which includes segmented worms.

2. **Most Prominent Feature**: The most prominent feature of the phylum Annelida is that they are true coelomates. This means they possess a true body cavity (coelom) that is completely lined by mesoderm.

3. **Segmentation**: Annelids exhibit true segmentation, which means their bodies are divided into repeated segments both externally and internally.

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