Respiratory tree occurs in

Options

A. Holothuroidea
B. Echinoidea
C. Ophiuroidea
D. Crinoidea.

Answer

### Step-by-Step Solution:

1. **Identify the Class of Echinoderms**: The question asks about the respiratory tree, which is a specific structure found in certain echinoderms. Echinoderms are classified into different classes, including Asteroidea, Echinoidea, Holothuroidea, and others.

2. **Focus on Holothuroidea**: Among these classes, the class Holothuroidea, which includes sea cucumbers, is known to possess a respiratory tree. This is a unique feature of this class.

3. **Understand the Function of the Respiratory Tree**: The respiratory tree in sea cucumbers is located in the cloaca. It functions in respiration by drawing water in and expelling it through the anus, allowing for gas exchange.

4. **Differentiate from Other Classes**: Other classes of echinoderms, such as Asteroidea, Echinoidea, and Crinoidea, do not have a respiratory tree. Instead, they typically respire through a water vascular system or ambulacral system.

5. **Conclusion**: Therefore, the respiratory tree occurs specifically in the class Holothuroidea, which includes sea cucumbers.

### Final Answer:
The respiratory tree occurs in **Holothuroidea** (sea cucumbers).

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