These worms have not widely diversified since the Cambrian. They were diversified in the Cambrian. Darwin presents a cone-like development of diversity in his theory, but certain data from evolution such as this presentation implies that various areas of evolution may be cylindrical in development.-http://www.arn.org/blogs/index.php/literature/2012/10/11/priapulids_challenge_the_cone_of_increas-More evidence of long term stasis of oceanic forms:-http://www.sciencedaily.com/releases/2012/10/121010172116.htm