Which one of the following is not an echinoderm?

Options

A. Sea lily
B. Sea urchin
C. Sea pen
D. Sea cucumber

Answer

To determine which of the following options is not an echinoderm, we will analyze each option based on their characteristics and classification.

### Step-by-Step Solution:

1. **Understand Echinoderms**: Echinoderms are a group of marine animals characterized by their spiny skin, radial symmetry, and a water vascular system. Common examples include sea stars, sea urchins, and sea cucumbers.

2. **Evaluate Option 1: Sea Lily (Antedon)**:
- Sea lilies are indeed echinoderms. They have a spiny body and branched arms, which are typical features of echinoderms.
- **Conclusion**: Sea lily is an echinoderm.

3. **Evaluate Option 2: Sea Urchin (Echinus)**:
- Sea urchins also belong to the echinoderm group. They have a spiny exterior but lack arms, which is consistent with echinoderm characteristics.
- **Conclusion**: Sea urchin is an echinoderm.

4. **Evaluate Option 3: Sea Pen (Penaeidae)**:
- Sea pens are not echinoderms; they belong to the phylum Cnidaria (which includes corals and jellyfish). They do not have the spiny skin or other characteristics typical of echinoderms.
- **Conclusion**: Sea pen is NOT an echinoderm.

5. **Evaluate Option 4: Sea Cucumber (Cucumaria)**:
- Sea cucumbers are also echinoderms. They have a soft body and lack arms, but they still fit within the echinoderm classification due to their spiny skin.
- **Conclusion**: Sea cucumber is an echinoderm.

### Final Answer:
The option that is not an echinoderm is **Option 3: Sea Pen (Penaeidae)**.

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