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Abstract DEVELOPMENT AND EVOLUTION OF THE MUSCULATURE IN SIPUNCULAN WORMS Anja Schulze1 and Mary E. Rice2 The taxonomy of the Sipuncula, a small phylum of benthic marine
worms, is partially based on the number and degree of fusion of
the introvert retractor muscles as well as the arrangement of the
body wall musculature. The majority of sipunculan species develop
indirectly, passing through a lecithotrophic trochophore and a planktotrophic
pelagosphera larval stage. Some species show abbreviated development
in which the pelagic phase may be completely skipped. Here we examine
myogenesis in four species that represent different developmental
modes, using F-actin staining with fluorescent-labeled phalloidin
in conjunction with confocal laser scanning microscopy. All examined
species have smooth body wall musculature as adults and less than
the full set of four introvert retractor muscles. All go through
stages with four introvert retractor muscles that eventually fuse
into the reduced number in the adult. The circular and sometimes
the longitudinal body wall musculature is split into bands that
later fuse to form a smooth sheath. We have also reconstructed the
ancestral states of the introvert and body wall musculature using
Bayesian statistics. Our reconstructions suggest with high probability
that the ancestral sipunculan had four introvert retractor muscles,
longitudinal musculature split into bands and a smooth sheath of
circular body wall musculature. We conclude that the plesiomorphic
condition in sipunculans is four introvert retractor muscles. This
condition is retained in the larvae of all sipunculan species examined
in this study and by previous authors. We also found that crawling
larvae have more strongly developed body wall musculature than swimming
larvae which propel themselves by means of their metatrochal cilia.
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