Zoology 10th Edition by Stephen Miller,John Harley

Exercise 2

Annelida is monophyletic. Chaetopteridae and Sipuncula are basal annelids that lie outside of the Errantia and Sedentaria. Segmentation within the Annelida has been lost or modified within basal and some derived annelid taxa. The Clitellata evolved from ancestral sedentarians that invaded fresh water.
Discuss the statement that segmentation is a definitive, yet variable, characteristic of the Annelida.