The cell wall is composed of two thin overlapping shells which fit together like a soap case in

Options

A. desmids
B. Diatoms
C. dinoflagellates
D. slime moulds.

Explanation

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks about a type of organism whose cell wall is composed of two thin overlapping shells that fit together like a soap case.

2. **Analyzing the Options**: We need to evaluate the given options to identify which organism has this characteristic.

3. **Identifying the Correct Organism**: Upon reviewing the options, we find that diatoms are known for having a unique cell wall structure.

4. **Describing Diatoms' Cell Wall**: The cell wall of diatoms is made up of two overlapping shells called frustules. These frustules are composed of silica, which gives them a glass-like appearance.

5. **Understanding Frustule Structure**: The frustule consists of two parts: the apotheca (the upper part) and the hypotheca (the lower part). These two parts fit together, resembling a soap case.

6. **Conclusion**: Based on the analysis, the correct answer to the question is that the cell wall composed of two thin overlapping shells that fit together like a soap case is found in diatoms.

### Final Answer:
The correct answer is **diatoms**.

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