Which one of the following is unique to Mollusca?
(a) Canal system
(b) Mantle cavity
(c) Pseudocoelom
(d) Collar cells

Options

A. Canal system
B. Mantle cavity
C. Pseudocoelom
D. Collar cells

Answer

To solve the question "Which one of the following is unique to Mollusca?", we need to analyze each option provided:

1. **Canal system (Option A)**: This is not unique to Mollusca. Canal systems are found in various organisms, particularly in certain types of sponges and cnidarians.

2. **Mantle cavity (Option B)**: The mantle cavity is indeed a unique feature of Mollusca. It is a space between the mantle (a significant tissue layer) and the body of the mollusk, which plays a crucial role in respiration and excretion.

3. **Pseudocoelom (Option C)**: A pseudocoelom is a body cavity that is not completely lined by mesoderm. This feature is characteristic of certain invertebrates, such as nematodes, and is not unique to Mollusca.

4. **Collar cells (Option D)**: Collar cells, or choanocytes, are found in sponges and are not a feature of Mollusca.

Based on this analysis, the correct answer is **(b) Mantle cavity**, as it is a unique feature of the Mollusca phylum.

### Step-by-Step Solution:
1. **Identify the question**: Determine what is being asked about Mollusca.
2. **Analyze each option**:
- Option A: Canal system - Not unique to Mollusca.
- Option B: Mantle cavity - Unique to Mollusca.
- Option C: Pseudocoelom - Not unique to Mollusca.
- Option D: Collar cells - Not found in Mollusca.
3. **Determine the correct answer**: Based on the analysis, the mantle cavity is the unique feature.