All Sipunculids have at least three muscle systems: circular body wall muscles, introvert retractor muscles, and intestinal fasteners. This said, the digestive tubes and meta-nephridia also contain some muscle fibers (Cutler, 1994). Figure 8 depicts a frontal plane section of P. nigrescens found in the intertidal headlands of New South Whales.

            Sipunculid circular body wall muscles consist of two layers of smooth catch muscle. These include an outer layer of circular fibers, and an inner layer of longitudinal fibers, which are gathered into bands – this holds true for most species of the Phascolosoma genus (Saiz, 1993). Both body wall muscle layers can be seen in Figure 9 alongside introvert retractor muscle fibers. P. nigrescens has no diagonal muscles, but some larger species of Sipunculids often do (Cutler, 1994). The function of the introvert retractor muscles is to extend the body, and is imperative for rock-boring sipunculids, as these are used to squeeze the body and allow the intrusion of crevices in coral rock. As these muscle layers surround a fluid filled coelom, the musculature can act as a hydrostatic skeleton (Saiz, 1993).

            Introvert retractor muscles attach just behind the cerebral ganglia, and in P. nigrescens originate in the body wall 45% of the distance to the posterior end of the trunk (Cutler, 1994). As is common in most Phascolosoma, P. nigrescens has two pairs of retractor muscles (dorsal and ventral), and these can be seen in Figure 8. Figure 10 shows a close up of the posterior end of one of the retractor muscles. The purpose of these muscles is to withdraw the introvert. In doing so, sipunculids can protect their delicate tentacular crown from predation and damage (Saiz, 1993).

            The intestinal fasteners are present in most sipunculids, and usually develop during the pelagosphera phase (Jamieson, 1999). These are used to hold the gut coils in position, and are made up of long, thin spindle muscle. The intestinal fasteners were not observed in the sectioning of P. nigrescens from New South Whales; this was due to difficulties inherent in sectioning. Normally, the intestinal fasteners are near the anus or near the distal portion of the rectum, and are fixed anteriorly to the body wall. The spindle muscle lays along the rectum and is surrounded by the center of the gut coil. Individual intestinal coils also attach from small branches on the muscle (Cutler, 1994).

            Studies of electrolyte distribution in Phascolosma muscle by H.B. Steinbach (1940) suggest Phascolosoma muscles become inflated when transferred into sea water. In addition, sodium and chloride, normally excluded from muscle tissue, are often present in swollen muscles and are controlled by certain chemical and physical factors, which have yet to be identified. Muscle contraction requires the interaction of actin and myosin (Cutler, 1994). No studies have been done on whether muscular contraction in P. nigrescens is controlled primarily by myosin or calcium ions, but studies of P. scolops suggest it is myosin controlled (Cutler, 1994). It may, therefore, be reasonable to suggest myosin controlled musculature contraction in P. nigrescens. There is concern in constancy in sipunculid muscle biochemistry studies, as results have proven highly variable (Hooper et al., 2008).

            It has also been suggested in early muscle-nerve studies that circular body wall muscles, and introvert retractor muscles are controlled by the nervous system (Uexkull, 1903, Baglioni 1905). When the ventral nerve cord is electrically stimulated, slow impulses are sent throughout the body wall, but the retractor muscles do not respond. This said, retractor muscles respond to stimulation of the tentacles or introvert.

            Originally, it was believed that Sipunclids moved with as much swiftness as their Annelid cousins - moving by attaching the tentacular crown to the substrate and pulling the trunk forward, or with the undulating pattern characteristic of errant polychaetes (Hyman, 1959). Although this is somewhat true, the efficiency with which they move was greatly overstated. Due to limited muscle complexity, Sipunculids (especially rock boring species such as P. nigrescens) have very limited mobility outside of the substrate.

            A study was conducted at The University of Queensland to identify different movement forms of P. nigrescens. Four different P. nigrescens were removed from their burrows inside coral rubble, and placed in a plastic tank with fine sedimented sand. A coral fragment typical of the New South Whales headlands was placed 150cm away (furthest distance possible) to entice P. nigrescens to move towards it. It was assumed that having been dislodged from its preferred environment, P.nigrescens would make an effort to return to a secluded environment inside the coral rubble. Individuals were monitored for 30 minutes, and observations of their motions and distance were made every two minutes. Four possible locomotion strategies are suggested: “Inchworm-like’, ‘Push-off”, ‘Side-pull’,& ‘Spin’ (Figure 11). 

          
            When P. nigrescens is removed from its normal habitat, and placed into a plastic container with seawater but without sediment, it fully extends its introvert, and only periodically partially retracts it. When placed into the same container with a second P. nigrescens, both find each other and intertwine their bodies around the other.
It is suggested that P. nigrescens possesses very minimal locomotion potential outside of its burrow. The three existing muscle systems provide P. nigrescens with a very limited array of movement. Circular body wall muscles only help in elongating the introvert, longitudinal muscles allow movement of the introvert in pivoting from the base, and he introvert retractor muscles only serve, as the homologue implies, in retracting the introvert. Movements identified are simply the products of alternating the use of these three muscle systems. As the sample size of the study was small, it has yet to be determined which movements represent legitimate attempts at motion, and which are the biproducts of random movement of the introvert. Regardless, it is understood that the limitations in movement on most sipunculids are imposed by their lack of muscle complexity. Some Sipunculids have diagonal muscles, but this is not the norm (Cutler, 1994). The youtube video below shows the attempted “Inchworm-like” movement described in Figure 11. This occurs around 38 seconds in.