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Abstract An animal's ability to regrow lost tissues or structures can vary greatly during its life cycle. The annelidCapitella teletaexhibits posterior, but not anterior, regeneration as juveniles and adults. In contrast, embryos display only limited replacement of specific tissues. To investigate when during development individuals ofC. teletabecome capable of regeneration, we assessed the extent to which larvae can regenerate. We hypothesized that larvae exhibit intermediate regeneration potential and demonstrate some features of juvenile regeneration, but do not successfully replace all lost structures. Both anterior and posterior regeneration potential of larvae were evaluated following amputation. We used several methods to analyze wound sites: EdU incorporation to assess cell proliferation; in situ hybridization to assess stem cell and differentiation marker expression; immunohistochemistry and phalloidin staining to determine presence of neurites and muscle fibers, respectively; and observation to assess re‐epithelialization and determine regrowth of structures. Wound healing occurred within 6 h of amputation for both anterior and posterior amputations. Cell proliferation at both wound sites was observed for up to 7 days following amputation. In addition, the stem cell markervasawas expressed at anterior and posterior wound sites. However, growth of new tissue was observed only in posterior amputations. Neurites from the ventral nerve cord were also observed at posterior wound sites. De novoashexpression in the ectoderm of anterior wound sites indicated neuronal cell specification, although the absence ofelavexpression indicated an inability to progress to neuronal differentiation. In rare instances, cilia and eyes re‐formed. Both amputations induced expanded expression of the myogenesis geneMyoDin preexisting tissues. Our results indicate that amputated larvae complete early, but not late, stages of regeneration, which indicates a gradual acquisition of regenerative ability inC. teleta. Furthermore, amputated larvae can metamorphose into burrowing juveniles, including those missing brain and anterior sensory structures. To our knowledge, this is the first study to assess regenerative potential of annelid larvae. 

Abstract Spiralia is a large, ancient and diverse clade of animals, with a conserved early developmental program but diverse larval and adult morphologies. One trait shared by many spiralians is the presence of ciliary bands used for locomotion and feeding. To learn more about spiralian-specific traits we have examined the expression of 20 genes with protein motifs that are strongly conserved within the Spiralia, but not detectable outside of it. Here, we show that two of these are specifically expressed in the main ciliary band of the molluscTritia(also known asIlyanassa). Their expression patterns in representative species from five more spiralian phyla—the annelids, nemerteans, phoronids, brachiopods and rotifers—show that at least one of these,lophotrochin, has a conserved and specific role in particular ciliated structures, most consistently in ciliary bands. These results highlight the potential importance of lineage-specific genes or protein motifs for understanding traits shared across ancient lineages.