Which of the following groups of animals is having tube in tube body plan?

Options

A. Sponges
B. Coelenterates (cnidarians)
C. Aschelminthes (round worms)
D. Ctenophores

Explanation

To determine which group of animals has a tube-in-tube body plan, we need to analyze the options provided and understand the characteristics of their body plans.

### Step-by-Step Solution:

1. **Understand Tube-in-Tube Body Plan**:
- The tube-in-tube body plan refers to organisms that have a complete digestive system with two openings: a mouth for ingestion and an anus for egestion. This allows for a more efficient digestive process.

2. **Evaluate the Options**:
- **Sponges (Phylum Porifera)**:
- Sponges do not have a true body plan. They are porous and lack a digestive system with two openings. Therefore, they do not exhibit a tube-in-tube body plan.
- **Cnidarians (Phylum Cnidaria)**:
- Cnidarians, such as jellyfish and corals, have a body plan known as a blind sac. They possess only one opening that serves both as a mouth and an anus. Thus, they do not have a tube-in-tube body plan.
- **Aschelminthes (Phylum Aschelminthes)**:
- Aschelminthes, commonly known as roundworms, exhibit a tube-in-tube body plan. They have a complete digestive system with separate mouth and anus, making them the correct answer.
- **Ctenophores (Phylum Ctenophora)**:
- Ctenophores also have a body plan that features a single opening for both ingestion and egestion, similar to cnidarians. Therefore, they do not have a tube-in-tube body plan.

3. **Conclusion**:
- Based on the analysis, the only group of animals that exhibits a tube-in-tube body plan is **Aschelminthes** (roundworms).

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