Shell is internal in

Options

A. Monoplacophora
B. Gastropoda
C. Pelecypoda
D. Cephalpoda

Answer

To solve the question "Shell is internal in," we need to analyze the options provided: Monoplacophora, Gastropoda, Bivalvia (Pelecypoda), and Cephalopoda. Here's a step-by-step breakdown:

### Step 1: Understand the Classes of Mollusca
- All the options belong to the phylum Mollusca, which is known for its diverse forms and the presence of a calcareous shell in many species.

### Step 2: Analyze Monoplacophora
- Monoplacophora are primitive mollusks often referred to as "living fossils." They possess a single, external shell. Therefore, they do not fit the criteria of having an internal shell.

**Hint:** Look for characteristics of the shell in each class.

### Step 3: Analyze Gastropoda
- Gastropoda includes snails and slugs. They typically have a spiral shell that is external. This class does not have an internal shell.

**Hint:** Consider the common examples of each class and their shell types.

### Step 4: Analyze Bivalvia (Pelecypoda)
- Bivalvia, also known as Pelecypoda, includes organisms like mussels and oysters. They have two external shells (valves) and therefore do not have an internal shell.

**Hint:** Think about how many shells each class has and their arrangement.

### Step 5: Analyze Cephalopoda
- Cephalopoda includes squids, octopuses, and cuttlefish. This class is unique because they can have an internal shell (like the cuttlefish) or may lack a shell entirely. Thus, they fit the criteria of having an internal shell.

**Hint:** Identify which class has members with an internal shell or no shell at all.

### Conclusion
Based on the analysis, the correct answer to the question "Shell is internal in" is **Cephalopoda**.

### Final Answer
**Cephalopoda** (e.g., cuttlefish).