Fig 6. Rejection, support and new hypotheses on the evolutionary transformation of the segmental chaetal pattern characteristic for Capitellidae and most other Annelida (A) into that of Echiura (B, C).

The number of the ventral chaetal sacs and position of the growth zone supports the first hypothesis by suggesting that the ventral chaetae evolved from neuropodia by reduction of the number of chaetae and loss of the neuropodia. All other hypotheses are rejected by the results of this paper. A new hypothesis (hypothesis 5) is developed for the evolution of the anal chaetae in Urechidae and Echiurinae, according to which the anal chaetae evolved by multiplication of ventral chaetae.