Consider the phylogeny below. The coelom evolved once in the common ancestor of protostomes and deuterostomes. What does the phylogeny indicate about the lack of a coelom in flatworms (Platyhelminthes)

Flat worms belong to Phylum - Platyhelminthes as they have flat body because they don't have any coelom in their body or called acoelomate

Explanation:

Coelom is called as the true body cavity that is found in Porifera and Cnidaria but when it comes to platyhelminthes, the coelom is not found and thus they have flat like structure and they are categorized under first triploblastic animals that is they contain 3 tissue layer which are found in higher animals (Ectoderm, Mesoderm and endoderm) also cephalization takes place in this parasitic group which indicates evolution towards higher, complex organisms.