A simplified annelid phylogeny (Weigert and Bleidorn, 2016) summarizes the main annelid species investigated for segment formation and teloblasts. The number of segments appearing during the pelagic larval phase (but patterned during embryogenesis) and the number of segments added posteriorly after the start of benthic life can vary considerably (Balavoine, 2014). In leeches, a fixed number of segments is made during direct embryonic development and none added after hatching. In non-leech clitellates (such as Tubifex), the embryos make a few tens of segments during embryogenesis and add many more after hatching. In non-clitellate annelids (Errantia, Sedentaria, and early branching), the number of larval segments is variable, and many more are added in post-larval development: In Nereidids (Errantia) such as Platynereis, only three larval segments develop, while in Capitella (Sedentaria), 13 larval segments (Thamm and Seaver, 2008) and in Chaetopterus (an early-branching annelid) 15 larval segments develop (Seaver et al., 2001). Embryonic and post-embryonic teloblasts differ in the way they have been evidenced. Embryonic teloblasts in the Clitellates (Weisblat and Kuo, 2014; Goto et al., 1999a; Nakamoto et al., 2011) and in Platynereis (this work) have been directly observed in live specimens. By contrast, post-embryonic teloblasts are inconspicuous cells that are identified only so far by their molecular stem cell signature (Gazave et al., 2013; Dill and Seaver, 2008; Özpolat et al., 2016). No direct observation of post-embryonic teloblast patterns of division is available so far. No direct evidence for embryonic teloblasts giving birth to post-embryonic teloblasts exists in any species so far to our knowledge. The ancestor of annelids presumably had both larval and post-larval segments, and thus it raises the question of the presence of both embryonic and post-embryonic teloblasts in this ancestral annelid, and even more broadly in other spiralians such as chitons (a type of segmented mollusk). Structures shown in the figure are color coded and explained in the ‘symbol key’.