True coelom originated firstly in ………… .

Explanation

**Step-by-Step Solution:**

1. **Understanding Coelom**: Coelom is defined as a body cavity that is completely lined by mesoderm. It is important to distinguish between different types of body cavities, such as acoelomate (no body cavity), pseudocoelomate (body cavity not fully lined by mesoderm), and coelomate (true coelom).

2. **Identifying True Coelom**: A true coelom is characterized by the presence of a body cavity that is entirely surrounded by mesodermal tissue. This allows for the development of complex organs and systems within the cavity.

3. **Origin of True Coelom**: The first organisms to exhibit a true coelom belong to the phylum Annelida. Annelids are segmented worms, which include earthworms and leeches, and they possess a true coelom that allows for greater body organization and complexity.

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