Periplaneta has mosaic vision. Each ommatidium is composed of following parts except

Options

A. Corneal lens
B. Refractive crystalline cone
C. Rhabdome
D. Phaosome

Answer

To solve the question regarding the components of an ommatidium in Periplaneta (the scientific name for cockroach), we will analyze the parts typically found in an ommatidium and identify which one is not included.

### Step-by-Step Solution:

1. **Understand the Structure of an Ommatidium**:
An ommatidium is a functional unit of the compound eye found in arthropods, including cockroaches. Each ommatidium is responsible for capturing light and contributing to the overall image perceived by the organism.

2. **Identify the Components of an Ommatidium**:
The typical components of an ommatidium include:
- **Corneal Lens**: This is the outermost part that helps to focus light.
- **Refractive Crystalline Cone**: This part helps to direct the light towards the photoreceptors.
- **Rhabdom**: This is a light-sensitive structure that contains photoreceptor cells.
- **Retinula Cells**: These are the actual photoreceptor cells that convert light into nerve signals.

3. **Examine the Options Given**:
The options provided in the question are:
- A. Corneal lens
- B. Refractive crystalline cone
- C. Rhabdom
- D. Fossa

4. **Determine Which Part is Not Present**:
From the components listed above, we can see that the first three options (corneal lens, refractive crystalline cone, and rhabdom) are indeed parts of an ommatidium. However, the term "fossa" does not correspond to any component of an ommatidium. Instead, it refers to a depression or pit and is not involved in the structure of the ommatidia.

5. **Conclusion**:
Therefore, the part that is not present in the ommatidium of Periplaneta is **D. Fossa**.

### Final Answer:
**The correct answer is D. Fossa.**