Which of the following cells are present only in the epidermis of Hydra?
(A) Interstitial cells
(B) Cnidoblasts
(C) Sensory cells
(D) Germ cells

Options

A. B and C
B. B only
C. B and D
D. A,B,C and D

Explanation

To determine which cells are present only in the epidermis of Hydra, we will analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding the Epidermis of Hydra**:
- The epidermis is the outermost layer of the Hydra's body wall. It is crucial to identify which cells are specifically located in this layer.

2. **Option A: Interstitial Cells**:
- Interstitial cells are found inside the body of Hydra, not in the epidermis. They are located between the epidermis and the gastrodermis.
- **Conclusion**: Incorrect.

3. **Option B: Cnidoblasts (Nidoblasts)**:
- Cnidoblasts, also known as nidoblasts, are specialized cells that contain nematocysts (stinging structures). These cells are located in the epidermis and play a role in defense and capturing prey.
- **Conclusion**: Correct.

4. **Option C: Sensory Cells**:
- Sensory cells are involved in the perception of stimuli and are typically associated with nerve fibers. They are not specifically located in the epidermis but are found deeper within the organism.
- **Conclusion**: Incorrect.

5. **Option D: Germ Cells**:
- Germ cells, which develop into gametes, are also found in the epidermis of Hydra. They are involved in reproduction and can be seen swelling to form gonads.
- **Conclusion**: Correct.

6. **Final Answer**:
- The cells present only in the epidermis of Hydra are **B (Cnidoblasts) and D (Germ cells)**. Therefore, the correct answer is **B and D**.