All the following statement are correct about Aves but one is wrong which one is wrong ?

Options

A. Fertilisation is internal
B. They are oviparous and development is direct, without exception
C. They are homeothermal animals with twelve pairs of cranial nerves
D. All the flightless birds have sternum with keel

Answer

To determine which statement about Aves (birds) is incorrect, we will analyze each statement provided in the question.

### Step-by-Step Solution:

1. **Understanding Aves**: Aves is a class of vertebrates that includes all birds. They are characterized by certain features that distinguish them from other classes.

2. **Statement 1: Fertilization is internal**:
- In birds, fertilization occurs inside the female's body. The male and female gametes unite internally before the eggs are laid.
- **Conclusion**: This statement is correct.

3. **Statement 2: They are oviparous and development is direct without exception**:
- Birds are oviparous, meaning they lay eggs. Additionally, they exhibit direct development, which means that there is no larval stage; the young birds hatch directly from the eggs.
- **Conclusion**: This statement is correct.

4. **Statement 3: They are homeothermal animals with 12 pairs of cranial nerves**:
- Homeothermal means that birds maintain a constant body temperature. Birds, along with reptiles and mammals, have 12 pairs of cranial nerves.
- **Conclusion**: This statement is correct.

5. **Statement 4: All the flightless birds have sternum with keel**:
- The keel is a structure on the sternum that serves as an attachment point for flight muscles. Flightless birds, such as ostriches and emus, do not have a keel because they do not need the adaptations for flight.
- **Conclusion**: This statement is incorrect.

### Final Answer:
The incorrect statement about Aves is: **"All the flightless birds have sternum with keel."**

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