The arrangement of the body-wall musculature and supporting mesenchyme of the bipinnaria larva of Pisaster ochraceus is described, based on an EM analysis of Pisaster ochraceus larvae. There are four main cell types: (i) a generalized mesenchyme that links the ectoderm with internal structures, e.g. coelom and gut; (ii) dorsal longitudinal muscles, which flex the body; (iii) oral-field muscles, which flex the larval lobes individually; and (iv) cords of subtrochal cells, a distinctive type of mesenchyme located beneath the ciliary band. Processes from the subtrochal cells insert into the band and associate closely with the ciliary nerve. The function the subtrochal cells perform is not obvious, but a role in body support is proposed, with the cells acting as tensile elements to increase the structural stability of the band. The insertions formed by the subtrochal cells could be attachment structures or, assuming the cells are contractile, they could be innervation sites. The larval lobes in the bipinnaria are arranged in series along the body, and the muscles that supply them are also arranged in series. Similar patterns of repeating elements occur in other echinoderm larvae, in the hemichordate tornaria larva, and also in chordates. In particular, there are marked similarities between (i) the arrangement of the oral-field muscles in the bipinnaria and the muscle bands of pelagic tunicates, and (ii) the subtrochal cells in the bipinnaria and the myotomes in Amphioxus. These similarities are suggestive; whether they are due to homology is an open question. On the assumption that they may, a hypothesis is proposed to explain the differences between anterior and posterior mesodermal repeats in vertebrates (somitomeres and somites) based on differences between anterior and posterior lobes in the bipinnaria.