Which of the following organisms is not radially symmetrical ?

Options

A. Pila
B. Echinus
C. Cucumaria
D. Ctenoplane

Answer

To determine which of the given organisms is not radially symmetrical, we need to understand what radial symmetry is and identify the phylum to which each organism belongs.

### Step-by-Step Solution:

1. **Understand Radial Symmetry**:
- Radial symmetry means that an organism can be divided into similar halves by multiple planes passing through a central axis. This type of symmetry is commonly found in certain animal phyla.

2. **Identify the Organisms and Their Phyla**:
- **Paila**: This organism belongs to the phylum **Mollusca**.
- **Echinus**: This organism belongs to the phylum **Echinodermata**.
- **Cucumeria**: This organism also belongs to the phylum **Echinodermata**.
- **Tenoplana**: This organism belongs to the phylum **Tenophora**.

3. **Determine Symmetry Based on Phylum**:
- **Mollusca**: Organisms in this phylum do not exhibit radial symmetry; they are often bilaterally symmetrical.
- **Echinodermata**: Organisms in this phylum exhibit radial symmetry, especially in their adult forms.
- **Tenophora**: Organisms in this phylum also exhibit radial symmetry.

4. **Conclusion**:
- Since **Paila** belongs to the phylum Mollusca, which does not exhibit radial symmetry, it is the organism that is not radially symmetrical.

### Final Answer:
The organism that is not radially symmetrical is **Paila**.