Which one of the following groups of animals is bilaterally symmetrical and triploblastic

Options

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

Explanation

To determine which group of animals is bilaterally symmetrical and triploblastic, we can analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understand the Terms**:
- **Bilateral Symmetry**: An organism is bilaterally symmetrical if it can be divided into two identical halves along one plane.
- **Triploblastic**: An organism is triploblastic if it has three germ layers: ectoderm, mesoderm, and endoderm.

2. **Evaluate Option 1: Cylindrata (Cnidaria)**:
- Cnidarians, such as jellyfish and corals, are primarily **diploblastic**, meaning they have only two layers (ectoderm and endoderm) with a non-cellular layer called mesoglia.
- They are not bilaterally symmetrical; they exhibit radial symmetry.
- **Conclusion**: This option is incorrect.

3. **Evaluate Option 2: Aschelminthes (Roundworms)**:
- Roundworms are indeed **bilaterally symmetrical** as they can be divided into two equal halves along one plane.
- They are also **triploblastic**, having ectoderm, mesoderm, and endoderm.
- **Conclusion**: This option is correct.

4. **Evaluate Option 3: Ctenophores**:
- Ctenophores (sea walnuts) are primarily **radially symmetrical**, meaning they can be divided into equal halves through multiple planes.
- They are also **diploblastic**, not triploblastic.
- **Conclusion**: This option is incorrect.

5. **Evaluate Option 4: Sponges**:
- Sponges are **asymmetrical** and do not have true tissues; they are also **diploblastic**.
- They cannot be divided into equal halves along any plane.
- **Conclusion**: This option is incorrect.

### Final Answer:
The correct group of animals that is bilaterally symmetrical and triploblastic is **Option 2: Aschelminthes (Roundworms)**.

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