Annelids, like many other invertebrate animals, replace lost body parts in a process called regeneration. However, the ability to regenerate lost segments is present in some groups and not others, for example leeches do not regenerate lost segments. Anterior and posterior regeneration involves the formation of a bud containing stem cells that differentiate into the new head or tail segments. Annelid regeneration also involves remodelling of surviving body fragments. The ability of annelids to regenerate tail segments appears to be nearly universal among species capable of regeneration. The ability to regenerate head segments, although common, is not universal and can depend on the number of segments lost. The absence and presence of regeneration across annelid groups, including closely related species, suggests that regeneration ability may be an ancient trait that has been lost in some species during annelid evolution. Why regeneration varies among annelid species remains an intriguing question for life scientists.

Key Concepts:

• Annelids vary in their capability for regenerating body segments, including among closely related species.
  
• The ability of annelids to regenerate posterior segments appears to be nearly universal.
  
• The ability of annelids to regenerate anterior segments, although common, is not universal and is often limited depending on the number of segments lost.
  
• Annelid regeneration may involve both epimorphic and morphallactic mechanisms.
  
• Multiple losses and gains of regeneration ability have likely occurred during annelid evolution.
  
• Why regenerative ability among annelids varies extensively remains unclear.