"Tube in a tube " plan is not exhibited by which one of the following phyla ?

Options

A. Coelenterata
B. Annelida
C. Aschelminthes
D. Arthropoda

Answer

To determine which phylum does not exhibit the "tube in a tube" body plan, we can analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understand the "tube in a tube" body plan**: This body plan refers to organisms that have a complete digestive system with two openings: a mouth and an anus. This allows for a more efficient digestion process.

2. **Analyze the options**:
- **a. Cnidaria (Cilindrates)**: Members of this phylum, such as jellyfish and corals, have a simple body plan with a single opening that serves as both mouth and anus. They do not exhibit the "tube in a tube" plan.
- **b. Annelida**: This phylum includes segmented worms like earthworms, which do have a complete digestive system with both a mouth and an anus, thus exhibiting the "tube in a tube" plan.
- **c. Echinodermata**: Echinoderms, such as starfish and sea urchins, also have a complete digestive system, hence they exhibit the "tube in a tube" plan.
- **d. Arthropoda**: This phylum includes insects, arachnids, and crustaceans, all of which possess a complete digestive system with both mouth and anus, fitting the "tube in a tube" description.

3. **Conclusion**: Since Cnidaria is the only phylum listed that does not have a complete digestive system (only one opening), the answer to the question is **option a: Cnidaria (Cilindrates)**.

### Final Answer:
The "tube in a tube" plan is not exhibited by **option a: Cnidaria (Cilindrates)**.

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