Which one first evolved in annelida

Options

A. Triploblastic development
B. Bilateral symmetry
C. Cephalisation
D. True coelom

Answer

To answer the question "Which one first evolved in annelida?" we need to analyze the options provided: Triploblastic development, Bilateral symmetry, Cephalization, and True coelom.

### Step-by-Step Solution:

1. **Understanding Triploblastic Development**:
- Triploblastic development refers to organisms that have three germ layers: ectoderm, mesoderm, and endoderm.
- This type of development is characteristic of more advanced organisms and was first seen in the phylum Platyhelminthes (flatworms), not in Annelida.
- **Conclusion**: This option is incorrect for Annelida.

2. **Understanding Bilateral Symmetry**:
- Bilateral symmetry means that the body can be divided into two identical halves along a single plane.
- This trait is also seen in other phyla, including Mollusca, before Annelida.
- **Conclusion**: This option is also incorrect for Annelida.

3. **Understanding Cephalization**:
- Cephalization is the concentration of sensory organs and nervous tissue at the anterior end of the organism.
- In Annelida, cephalization is not well developed compared to other phyla, and thus it cannot be said to have evolved in Annelida.
- **Conclusion**: This option is incorrect for Annelida.

4. **Understanding True Coelom**:
- A true coelom is a body cavity that is completely lined by mesoderm.
- Annelids are coelomate organisms, meaning they possess a true coelom, which is a significant evolutionary advancement in this phylum.
- True coelom is considered to have first evolved in Annelida, distinguishing them from earlier phyla that had pseudocoeloms.
- **Conclusion**: This option is correct for Annelida.

### Final Answer:
The feature that first evolved in Annelida is **True coelom**.

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