Retinal sheath is absent in 

Options

A. diurnal animals
B. nocturnal animals
C. both 1 & 2
D. none

Answer

To answer the question "Retinal sheath is absent in," we will analyze the options provided and the concept of retinal sheath in animals.

### Step-by-Step Solution:

1. **Understand the Concept of Retinal Sheath**:
- The retinal sheath is a bundle of cells or tissues that secrete a chemical known as retinal, which is essential for vision, especially in low-light conditions.

2. **Identify the Function of Retinal**:
- Retinal is an aldehyde-based compound secreted by rod cells in the eye. It plays a crucial role in dark vision, allowing animals to see in the absence of light.

3. **Differentiate Between Diurnal and Nocturnal Animals**:
- **Diurnal Animals**: These are animals that are active during the day and can see well in both daylight and darkness.
- **Nocturnal Animals**: These animals are active during the night and have adaptations that allow them to see in low-light conditions.

4. **Determine the Presence of Retinal Sheath**:
- The retinal sheath is present in both diurnal and nocturnal animals because both types of animals require retinal for vision, especially in low-light situations.

5. **Evaluate the Options**:
- The options provided are:
- A) Diurnal animals
- B) Nocturnal animals
- C) Both one and two
- D) None of these
- Since retinal sheath is present in both diurnal and nocturnal animals, options A and B are incorrect. Option C is also incorrect because retinal sheath is not absent in either group.

6. **Conclusion**:
- Therefore, the correct answer is option D: None of these, as retinal sheath is not absent in any of the mentioned categories.

### Final Answer:
The retinal sheath is absent in **none of these** (option D).