Basal socket muscle fibres of Barentsia gracilis are morphologically similar to invertebrate muscles classified as slow, tonic-type muscles. They are divisible into a non-contractile bulbous and a contractile myofilamentous portion, are innervated on their non-contractile surface, and are obliquely striated. The electron density and myofilament packing vary from fibre to fibre. Some muscle fibres appear degenerate in that they contain disorganized-appearing myofilaments and electron-dense bodies morphologically reminiscent of autophagosomes found in cells undergoing cytolysis.
A distinguishing feature of muscle fibres observed in B. gracilis is the presence of crystalloids, morphologically reminiscent of protein storage crystalloids found in other cell types. These crystalloids are seen to be composed of 30-70 Å thick, helically shaped filaments arranged in a hexagonal lattice. Filaments have a pitch of 130-150 Å and a tilt-angle of 35-45 degrees. Cross-links occur between adjacent filaments at regular intervals. Crystalloids appear to be formed in the bulbous portion of muscle cells. Constituent, 30-70 Å filaments appear to branch from crystalloids and are found free in the myofilamentous area of muscle fibres.
The above evidence, coupled with evidence elsewhere in the literature, suggests that the crystalloids may be a storage form of myofilament precursor which relate either to myogenesis or to cyclic regenerative and degenerative phenomena known to occur in Entoprocts, including B. gracilis.