The only fresh water animal in phylum cnidaria is________.

Explanation

**Step-by-Step Solution:**

1. **Understanding Phylum Cnidaria**: Phylum Cnidaria includes a diverse group of animals, primarily marine, such as jellyfish and sea anemones. They are characterized by their radial symmetry.

2. **Identifying Freshwater Species**: While most Cnidarians are marine, there is a notable exception that lives in freshwater environments.

3. **Recognizing the Freshwater Cnidarian**: The only freshwater animal in the phylum Cnidaria is the Hydra. This organism is unique because it thrives in freshwater habitats.

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