Which one is first evolved in annelida?

Options

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

Explanation

To determine which characteristic first evolved in Annelida, we need to analyze the options provided:

1. **Triploblastic Development**: This refers to organisms having three germ layers: ectoderm, endoderm, and mesoderm. While Annelids are triploblastic, this characteristic first evolved in Platyhelminthes (flatworms), not Annelida.

2. **Bilateral Symmetry**: This is a body plan in which the left and right sides of the organism are mirror images. Annelids do exhibit bilateral symmetry, but this trait is also present in other groups, such as Arthropods. Therefore, it is not unique to Annelida.

3. **Cephalization**: This is the concentration of sensory organs and nervous tissue at one end of the body, typically the head. Annelids do not exhibit significant cephalization; their body segments (metameres) are relatively equal and lack a specialized head region like that seen in vertebrates.

4. **True Coelom**: This refers to a body cavity that is completely lined by mesoderm. Annelids possess a true coelom, which is a significant evolutionary advancement. The true coelom is a characteristic that first evolved in this group.

Based on the analysis, the first characteristic to evolve in Annelida is **True Coelom**.

### Step-by-Step Solution:
1. **Identify the characteristics**: List the four options provided in the question.
2. **Analyze Triploblastic Development**: Note that this trait evolved earlier in Platyhelminthes and is not unique to Annelida.
3. **Evaluate Bilateral Symmetry**: Recognize that while Annelids do have bilateral symmetry, it is not a characteristic exclusive to them, as it appears in other groups.
4. **Consider Cephalization**: Understand that Annelids do not exhibit significant cephalization, as their body segments are similar and lack a specialized head.
5. **Conclude with True Coelom**: Identify that the true coelom is a defining feature of Annelids and is the first characteristic to evolve in this group.

### Final Answer:
The first characteristic to evolve in Annelida is **True Coelom**.