Why is coelenterata also called Cnidaria?

Explanation

### Step-by-Step Solution:

1. **Understanding the Phylum Coelenterata**: Coelenterata is a phylum that includes simple aquatic animals such as jellyfish, corals, and sea anemones. They are characterized by their radial symmetry and the presence of a central digestive cavity.

2. **Introduction to the Term Cnidaria**: The term "Cnidaria" is derived from the Greek word "knide," which means "nettle." This name reflects the presence of specialized cells that are unique to this group.

3. **Unique Cells - Nematocysts**: One of the defining features of Cnidarians is the presence of specialized cells called nematocysts (or cnidocytes). These cells contain a capsule that can eject a stinging thread when triggered.

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