Review articleThe polychaete-to-clitellate transition: An EvoDevo perspective

- The polychaete-to-clitellate transition was a major event in annelid evolution.
- Direct development emerged in the clitellate lineage.
- Pattern of cell fate segregation is conserved between polychaetes and clitellates.
- A heterochronic shift between cell lineages results in direct development.
- Modularity in axial patterning mechanism enabled the heterochronic change.