Which of the following is fixed

Options

A. Antedon
B. Asterias
C. Echinus
D. Cucumaria.

Answer

To solve the question "Which of the following is fixed?" we will analyze each option provided and determine which one is fixed to the substratum.

### Step-by-Step Solution:

1. **Understanding the Question**:
- The term "fixed" refers to an organism that is attached to a surface and does not move freely.

2. **Analyzing the Options**:
- **Option 1: Antedone (Feather Star)**:
- The feather star is known to be fixed to the substratum. It has a cup-shaped central disc and five slender arms but does not move from its location.
- **Option 2: Starfish**:
- Starfish are not fixed; they can move using their tube feet, which are part of their ambulacral system.
- **Option 3: Echinus (Sea Urchin)**:
- Sea urchins are also mobile and can move using their spines.
- **Option 4: Cucumeria (Sea Cucumber)**:
- Sea cucumbers are not fixed either; they can move around and are also known to respire through their cloaca.

3. **Conclusion**:
- After analyzing all the options, the only organism that is fixed to the substratum is the **Antedone (Feather Star)**. Therefore, the correct answer is **Option 1**.

### Final Answer:
- The fixed organism is **Antedone (Feather Star)**.