(A) Distribution, and ancestral state reconstruction, of midgut smooth muscles in Bilateria. Ancestral states were inferred using Parsimony and Maximum Likelihood (ML) (posterior probabilities indicated on nodes). Character states from: Chordata (Marieb and Hoehn, 2015), Echinodermata (Feral and Massin, 1982), Chaetognatha (Duvert and Salat, 1995), Mollusca (Royuela et al., 2000), Annelida (Anderson and Ellis, 1967), Priapulida (Carnevali and Ferraguti, 1979), Nematoda (White, 1988), Arthropoda (Goldstein and Burdette, 1971) and Tardigrada (Shaw, 1974). (B) Distribution, and ancestral state reconstruction, of cardiomyocyte ultrastructure in Bilateria. Ancestral states were inferred using Parsimony and ML (posterior probabilities indicated on nodes). Note that, due to the widespread presence of striated cardiomyocytes in bilaterians, the support value for an ancestral smooth ultrastructure in the ML method remain modest (0.56). This hypothesis receives independent support from the comparison of Cs (Figure 1B, Figure 7—figure supplement 1) and developmental data (see Discussion). Character states follow the review by Martynova (Martynova, 1995, 2004) and additional references for Siboglinum (Jensen and Myklebust, 1975), chordates (Hirakow, 1985), Peripatopsis (Nylund et al., 1988), arthropods (Tjønneland et al., 1987), Meiomenia (Reynolds et al., 1993) and Lepidopleurus (Økland, 1980). In Peripatopsis and Lepidopleurus, some degree of alignment of dense bodies was detected (without being considered regular enough to constitute striation), suggesting these might represent intermediate configurations.