Organization of trunk retractors, telotroch flexor, and telotroch constrictor in Phoronopsis harmeri larvae. A. Overview of two trunk retractors (tr) in the trunk coelom (SEM). B. Upper portion of the trunk retractor contacts the coelomic lining (cl) of the trunk coelom and collar ring muscle (cr) (SEM). C. Lower (posterior) portion of trunk retractor forms numerous long processes (white arrowheads) (SEM). D. Fine cross section of trunk retractor (TEM). Muscle cells connect to each other via desmosomes (black arrowheads). E. Middle portion of the trunk retractor contains nuclei of muscle cells. F. Cross section of larval trunk. Cells of the trunk coelom (c3) somatopleura form telotroch flexors and connect to each other via desmosomes (black arrowhead). G. Epidermal cells near the telotroch contain cross-striated-like myofilaments (closed arrowheads) in apical cytoplasm (TEM). Abbreviations: bc – blastocoel; bl – basal lamina; c3 – trunk coelom; mi – microvilli; ms – metasomal sack; n – nucleus.