Annelida have a true body cavity.

Answer

### Step-by-Step Solution:

1. **Understanding Annelida**: Annelida is a phylum that includes segmented worms such as earthworms, leeches, and polychaetes. They are known for their segmented bodies.

2. **Definition of True Body Cavity**: A true body cavity, also known as a coelom, is a fluid-filled space that is completely lined by mesoderm tissue. This is different from a pseudocoelom, which is only partially lined by mesoderm.

3. **Coelom Formation**: In annelids, the coelom is formed during embryonic development through a process called "splitting of the mesoderm." This means that the mesoderm, which is one of the three primary germ layers in the embryo, divides to create a cavity.

4. **Characteristics of Coelom**: The coelom in annelids is a true coelom, meaning it is fully lined with mesoderm. This cavity is filled with coelomic fluid, which helps in various physiological functions, including movement and the transport of nutrients.

5. **Function of Coelom**: The presence of a coelom allows for greater complexity in body structure and function. It provides a space for the development of internal organs and facilitates movement by allowing the body to change shape without affecting the digestive tract.

6. **Conclusion**: Therefore, it is correct to state that annelids possess a true body cavity, which is a characteristic feature of coelomate animals.