The myoanatomy of adult phoronids has never been comprehensively studied by fluorescent staining and confocal laser scanning microscopy. Because the organization of the musculature may provide insight into phoronid biology and phylogeny, phoronid myoanatomy warrants detailed investigation. The current study provides the first description based on the use of modern methods of the musculature of the very small phoronid Phoronis ovalis. The musculature of the lophophore base includes radial, longitudinal, and circular muscles; pharynx dilators; and paired lateroabfrontal muscles. The musculature of the anterior part of the body is formed by outer-circular, middle-diagonal, and inner-longitudinal muscles; because all of the cells in these muscles contact the basal lamina, the musculature in the anterior part of the body forms a single layer. In the posterior part of the body, diagonal muscles are absent, and the longitudinal musculature is represented by small, thin bundles. In the terminal end of the body, there is an inversion of circular and longitudinal muscles. The organization of the musculature in the lophophore base and anterior part of the body suggests that the lophophore can move in different directions in order to capture food from local water currents. The organization of the musculature of the terminal end would enable this part of the body to be used for digging into the substratum. The four-partitioned ground plan of the lophophoral musculature in P. ovalis and in bryozoans from all three main groups indicates the homology of the lophophore and the monophyly of the lophophorates as a united clade that includes three phyla: Phoronida, Bryozoa, and Brachiopoda. Some similarities in the organization of the lophophoral musculature, however, may reflect the similarities in the sessile life styles and feeding behaviors of P. ovalis and bryozoans.
The structure of the lophophore nervous system may help clarify the status of the clade Lophophorata, whose monophyly is debated. In the current study, antibody labeling and confocal laser scanning microscopy revealed previously undescribed main nerve elements in the lophophore in adult phoronids: Phoronis australis and Phoronopsis harmeri. In both species, the nervous system includes a dorsal ganglion, a tentacle nerve ring, an inner nerve ring, intertentacular groups of perikarya, and tentacle nerves. The dorsal ganglion and tentacle nerve ring contain many serotonin-like immunoreactive perikarya of different sizes. The inner nerve ring is described for the first time in adult phoronids with complex lophophore. It contains a thin bundle of serotonin-like immunoreactive neurites. The tentacles possess abfrontal, frontal, and laterofrontal nerves. The abfrontal nerves originate from the tentacle nerve ring; the frontal tentacle nerves extend from the inner nerve ring in P. harmeri and from the intertentacular frontal nerves in P. australis. The intertentacular groups of perikarya are found in phoronids for the first time. These small nerve centers connect with neither the tentacle nerve ring nor the inner nerve ring, giving rise to the laterofrontal tentacle nerves. The discovery of the inner nerve ring in adult phoronids makes the architecture of the lophophore nervous system similar in all lophophorates and thereby supports the monophyly of this group. The presence of intertentacular nerves, perikarya, and groups of perikarya is a typical feature of the nervous system in lophophorate presumably coordinating movements of the tentacles and thereby increasing the efficiency of lophophore functioning.