Evolution & Systematics

The origin of turbellarians is one of the numerous evolutionary questions that are still under debate. Would the simplicity of the body be a primitive or a derived trait? Some biologists believe that turbellarians and cnidarians have a common ancestor, which place them in a basal position. Others believe that flatworms have evolved from a coelomate ancestor by pedomorphosis (evolve from a early developmental stage, e.g. larvae) or reduction (simplification of the ancestor’s body) (Ruppert et al, 2004).