Which of the following phylum have "Tube within tube" body plan?

Options

A. Platyhelminthes
B. Coelenterata
C. Porifera
D. Nemathelminthes

Answer

To determine which phylum has a "tube within a tube" body plan, we can analyze the options provided step-by-step.

### Step-by-Step Solution:

1. **Understanding the "Tube within Tube" Body Plan**:
- The "tube within a tube" body plan refers to organisms that have a complete digestive system with two openings: a mouth for food intake and an anus for waste removal. This allows for a more complex and efficient digestive process.

2. **Analyzing Option 1: Platyhelminthes**:
- Platyhelminthes, or flatworms, have a simple body plan with a single opening that serves both as the mouth and anus (a gastrovascular cavity). Therefore, they do not exhibit a "tube within a tube" body plan.

3. **Analyzing Option 2: Cnidaria**:
- Cnidarians, such as jellyfish and corals, also have a single opening for both food intake and waste removal. Thus, they do not have a "tube within a tube" body plan.

4. **Analyzing Option 3: Porifera**:
- Porifera, or sponges, lack a true digestive system and do not have a mouth or anus. They filter feed through their porous bodies, so they do not fit the "tube within a tube" description.

5. **Analyzing Option 4: Nematoda**:
- Nematodes, or roundworms, possess a complete digestive system with a distinct mouth and anus. This structure exemplifies the "tube within a tube" body plan, making this option the correct answer.

### Conclusion:
The phylum that exhibits the "tube within a tube" body plan is **Nematoda** (Option 4).