Summary of the juvenile traits formation in sea urchins. (A) The formational modes of the primary pore canal (PPC). Four temnopleurid species, T. hardwikii, T. toreumaticus, T. reevesii, and M. globulus form PPCs on both sides and then the right PPC degenerates. T. hardwikii and T. toreumaticus form the PPCs with left-right symmetry, whereas T. reevesii and M. globulus form the PPCs asymmetrically at first and they then migrate symmetrically. Other species, including H. purcherrimus forms only a left PPC according to the present study and previous reports (Hara et al. 2003; Kitazawa et al. 2012). (B) A summary of the adult rudiment formation via formation of the cell mass (CM), based on the present study and previous reports (Fukushi 1959
1960; Kitazawa et al. 2012). When the PPCs are forming, about six cells of the oral ectoderm between the left post-oral arm and the oral lobe invaginate. The CM forms a hollow pouch and then a small process detaches from the most anterior part. The CM contributes to the adult rudiment. (C) A hypothetical scheme of evolutionary changes in the formation of the adult rudiment showing only the main orders. The phylogenetic relationships are based on Smith (1984) and Kroh and Smith (2010), but the branch lengths do not indicate time. After divergence from the Cidaroidea, the Euechinoidea evolved to form the adult rudiment from the amniotic cavity, whereas the indirect-developing Temnopleuridae changed the dependence of adult rudiment formation from the amniotic cavity to the CM.