Respiratory tree occurs in

Options

A. Sea Fan
B. Sea Lily
C. Sea Cucumber
D. Sec Urchin.

Answer

To solve the question "Respiratory tree occurs in," we will follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of the Respiratory Tree**:
- The respiratory tree is a part of the water vascular system found in certain echinoderms. It plays a crucial role in gas exchange, allowing the organism to inhale oxygen and exhale carbon dioxide.

**Hint**: Think about what the respiratory tree does and where it is located in the organism.

2. **Identify the Function of the Water Vascular System**:
- The water vascular system in echinoderms consists of various systems including the digestive, blood vascular, and respiratory systems. It aids in locomotion, feeding, and excretion.

**Hint**: Consider how the water vascular system is structured and its overall importance to echinoderms.

3. **Locate the Respiratory Tree in Echinoderms**:
- The respiratory tree consists of narrow tubes that branch from the digestive system, specifically near the anus. Water is drawn in through the anus, not the mouth, which is a unique feature of some echinoderms.

**Hint**: Focus on the anatomical location of the respiratory tree in relation to the digestive system.

4. **Identify Which Echinoderm Class Contains the Respiratory Tree**:
- The respiratory tree is a characteristic feature of the class Holothuroidea, which is commonly known as sea cucumbers.

**Hint**: Recall the different classes of echinoderms and identify which one is associated with the respiratory tree.

5. **Select the Correct Answer from the Options**:
- Given the options: sea fan, sea lily, sea cucumber, or sea urchin, the correct answer is **sea cucumber** (Holothuroidea).

**Hint**: Review the options and match them with the information about the respiratory tree.

### Final Answer:
The respiratory tree occurs in **sea cucumber**.