Cleidoic eggs are characteristic of :

Options

A. Insects
B. Fishes
C. Mammals
D. Reptiles and birds

Answer

### Step-by-Step Solution:

1. **Understanding Cleidoic Eggs**:
- Cleidoic eggs are characterized by having a hard outer membrane that protects the embryo from environmental changes. This is a key feature that distinguishes them from other types of eggs.

2. **Nutritional Content**:
- These eggs contain a significant amount of yolk, which provides essential nutrients to the developing embryo. This is important for the growth and development of the embryo inside the egg.

3. **Gas Exchange**:
- The eggshell of cleidoic eggs is permeable to gases, allowing for the exchange of oxygen and carbon dioxide. This is crucial for the embryo's respiration as it develops.

4. **Organisms that Lay Cleidoic Eggs**:
- Cleidoic eggs are primarily laid by reptiles and birds. While some insects also lay cleidoic eggs, they are generally smaller in size compared to those of reptiles and birds.

5. **Comparison with Other Organisms**:
- In contrast, fish typically lay non-cleidoic eggs, which do not have a hard outer membrane. Mammals, on the other hand, have various types of eggs (e.g., alicethal, microalecithal, mesolecithal) that differ in yolk content and structure.

6. **Conclusion**:
- Based on the characteristics and the organisms that lay cleidoic eggs, the correct answer to the question is that cleidoic eggs are characteristic of reptiles and birds.