................ is a unique system found in echinoderms.

Answer

**Step-by-Step Text Solution:**

1. **Identify the Subject**: The question asks for a unique system found in echinoderms. Echinoderms are a group of marine animals that include starfish, sea urchins, and sea cucumbers.

2. **Define Echinoderms**: The term "echinoderm" comes from Greek roots meaning "spiny skin." These organisms are characterized by their radial symmetry and a calcareous endoskeleton.

3. **Unique System in Echinoderms**: The unique system found in echinoderms is known as the **water vascular system**. This system is crucial for their movement, feeding, and respiration.

4. **Components of the Water Vascular System**: The water vascular system comprises several key structures:
- **Central Ring Canal**: This is the main canal that encircles the mouth of the echinoderm.
- **Radial Canals**: These extend from the central ring canal along each arm of the echinoderm.

5. **Function of the Water Vascular System**: The water vascular system functions by circulating water through the canals, which helps in:
- **Locomotion**: By controlling the tube feet, echinoderms can move.
- **Gas Exchange**: It aids in the exchange of gases.
- **Nutrient Transport**: It helps in the distribution of nutrients throughout the body.

6. **Conclusion**: Therefore, the answer to the question is that the **water vascular system** is a unique system found in echinoderms.

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