Which of the following is not true about Annelida?

The correct option is Pseudocoelom.

To understand why this statement is not true about Annelida, let us analyze the characteristic features of the phylum Annelida:
1. Coelom Type: Annelids are true coelomate animals. They possess a true coelom (schizocoelom), which is a fluid-filled body cavity completely lined by mesoderm. A pseudocoelom (false body cavity) is a characteristic feature of roundworms (phylum Aschelminthes/Nematoda), not annelids. Therefore, the statement that Annelida has a pseudocoelom is incorrect.
2. Ventral Nerve Cord: The nervous system of annelids typically consists of a double, solid ventral nerve cord running along the length of the body, which is a true statement.
3. Closed Circulatory System: Annelids possess a closed circulatory system where blood flows through a closed network of blood vessels. This is a true statement.
4. Segmentation: Annelids exhibit true metameric segmentation, meaning their body is divided externally and internally into repetitive segments or metameres. This is a true statement.

Since annelids possess a true coelom and not a pseudocoelom, "Pseudocoelom" is the correct option representing the statement that is not true about Annelida.