_____ are known to have pseudocoelom.

Explanation

**Step-by-Step Solution:**

1. **Understanding Pseudocoelom**:
- Pseudocoelom refers to a fluid-filled cavity that is not completely lined by mesoderm. It is found between the outer body wall and the internal organs.

2. **Identifying Organisms with Pseudocoelom**:
- Organisms that possess a pseudocoelom are typically classified under a specific phylum.

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