Amphibian skin is externally covered by

Options

A. placoid scales
B. cycloid scales
C. ctenoid scales
D. no scales

Answer

To answer the question "Amphibian skin is externally covered by", we will analyze the options provided and determine the correct answer step by step.

### Step-by-Step Solution:

1. **Understanding Amphibians**:
- Amphibians are a class of animals that can live both in water and on land. This dual habitat requires them to have certain adaptations.

**Hint**: Remember that amphibians include frogs, toads, and salamanders, which have unique skin characteristics.

2. **Skin Functionality**:
- The skin of amphibians plays a crucial role in respiration. They can absorb oxygen through their skin, which means their skin must remain moist.

**Hint**: Consider how amphibians breathe and the importance of moisture for their survival.

3. **Analyzing the Options**:
- **Placoid Scales**: These are found in sharks and are sharp and tooth-like. Since amphibians are not sharks, this option is incorrect.
- **Cycloid Scales**: These are smooth, overlapping scales found in fish like salmon and carp. Amphibians do not have these scales, so this option is also incorrect.
- **Tenoid Scales**: These are found in bony fishes, such as perch and sunfish. Again, amphibians do not possess these scales, making this option incorrect.
- **No Scales**: Given that amphibians need to keep their skin moist for respiration and do not have scales, this option is correct.

**Hint**: Eliminate the options that are associated with fish, as amphibians are distinct from them.

4. **Conclusion**:
- Based on the analysis, the correct answer is that amphibian skin is externally covered by **no scales**.

### Final Answer:
Amphibian skin is externally covered by **no scales**.