Which of the following statements is incorrett about sea walnuts?

Options

A. They are diploblastic animals
B. They exhibit tissue-level of organisation
C. They undergo indirect development
D. Fertilisation is internal

Explanation

To determine which statement about sea walnuts is incorrect, we will analyze the provided options based on the characteristics of sea walnuts.

### Step-by-Step Solution:

1. **Identify the Phylum**: Sea walnuts belong to the phylum **Ctenophora**. This is an important characteristic to note as it helps classify the organism correctly.

**Hint**: Remember the classification of organisms is crucial in understanding their biological features.

2. **Germ Layers**: Sea walnuts are **diploblastic**, meaning they have two germ layers (ectoderm and endoderm). This is a correct statement.

**Hint**: Diploblastic organisms typically have simpler body structures compared to triploblastic organisms.

3. **Level of Organization**: They exhibit a **tissue level of organization**, which is also a correct statement. This indicates that their cells are organized into tissues.

**Hint**: Understanding levels of organization (cellular, tissue, organ, system) helps in distinguishing between different types of organisms.

4. **Development Type**: Sea walnuts undergo **indirect development**, which involves a larval stage before reaching maturity. This statement is also correct.

**Hint**: Indirect development is common in many aquatic organisms, where they have a larval form that is different from the adult form.

5. **Fertilization Method**: The fertilization in sea walnuts is **external**, meaning that the eggs and sperm are released into the water where fertilization occurs. If an option states that fertilization is internal, this would be the incorrect statement.

**Hint**: Knowing the reproductive methods of organisms can help clarify their life cycles and ecological roles.

### Conclusion:
Based on the analysis, the incorrect statement about sea walnuts is that they undergo **internal fertilization**. The correct method is **external fertilization**.

### Final Answer:
The incorrect statement about sea walnuts is that they undergo internal fertilization.