Organization of the lophophoral nervous system in the cyclostome bryozoans confirms the relationship of the Bryozoa and Brachiozoa

Peculiarities of Tentacle Innervation of Flustrellidra hispida and Evolution of Lophophore in Bryozoa

The study of the lophophore organization is of great importance for the reconstruction of lophophorate phylogeny and for understanding the evolutionary transformation in each phylum of Lophophorata. The innervation of the lophophore in ctenostome bryozoan Flustrellidra hispida was studied using immunocytochemistry and confocal laser scanning microscopy. It has been demonstrated that this species has an outer nerve ring giving rise to the tentacle nerves. The outer nerve ring was earlier described in some ctenostomates and cyclostomates, but not as connected with nerves. The discovered feature of lophophore innervation in F. hispida suggests the evolutionary transformation from a hypothetical phoronida-like ancestor lophophore bearing a prominent outer nerve ring with numerous tentacle nerves emanating from it, to the complex bell-shaped lophophore of F. hispida with a well-pronounced outer nervous ring bearing a few tentacle nerves. The next one in this hypothetical row is the lophophore of the other ctenostomates and some cyclostomates with no ring-nerve connection and cheilostomates lophophore with no outer nerve ring at all.

Anatomy of the coelomic system in Novocrania anomala (Brachiopoda, Craniiformea) and relationships within brachiopods

Brachiopoda is a relict phylum of marine benthic animals that have not been adequately studied with modern microscopy methods. Microscopic study may provide useful information on the evolution of the brachiopod body plan and brachiopod phylogeny. Understanding the organisation of the coelomic system is important because of its role in body form and compartmentalisation. Most brachiopods are considered to have a bipartite coelomic system; the only known exception is Lingulida, which have a tripartite coelomic system. In the present study, we provide the first complete 3D reconstruction of the coelomic system in the craniide brachiopod Novocrania anomala (Müller, 1776). Its coelomic system consists of the following five main parts, which are entirely separated from each other: 1) a pair of large brachial canals; 2) a complex system of paired small brachial canals and a perioesophageal coelom; 3) frontal coelomic chambers; 4) a main trunk coelom, which includes several semi-detached muscular chambers and mantle sinuses; and 5) a pair of posterior adductors chambers. These results indicate that the coelomic system of N. anomala (and perhaps of other craniides) is complex and cannot be considered to be bipartite or tripartite. The frontmost part of the coelomic system is represented by a pair of frontal chambers, which are considered to be a part of the lophophore but which are derived from dorsal mantle fold extensions and thus may be a part of the trunk coelomic system. A number of similarities were discovered between craniiformean and rhynchonelliformean coelomic systems, including the prominent dorsal projections of the large brachial canals and the morphological features of the perioesophageal coelom. The complex subdivision of the N. anomala trunk coelom is explained by the location and function of muscles, and by the location of several mesenteries.

Novel data on the innervation of the lophophore in adult phoronids (Lophophorata, Phoronida)

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.

Myoanatomy of the Lophophore in Adult Phoronids and the Evolution of the Phoronid Lophophore

Confocal laser scanning microscopy was used to study the myoanatomy of the lophophore of three phoronids with different types of lophophore: Phoronis ijimai, Phoronis australis, and Phoronopsis harmeri. A four-part ground plan of the lophophoral musculature was detected in all three species and was previously reported for Phoronis ovalis. The ground plan includes (i) a circular muscle, (ii) longitudinal muscles of the tentacular lamina, (iii) groups of paired distal muscles of the tentacular lamina, and (iv) frontal and abfrontal muscles of the tentacles. In P. australis, the tentacular lamina contains strong abfrontal and numerous frontal muscles. Phoronis harmeri has an inner circular muscle and arch-like muscles. Among all studied phoronids, the four-part ground plan of the lophophoral musculature is least complex in P. ijimai, which has a horseshoe-shaped lophophore. The results suggest two possible scenarios by which the morphology of the phoronid lophophore has transformed over evolutionary time. According to the first scenario, the morphology of the ancestral horseshoe-shaped lophophore became more complicated in the case of most phoronids but became simplified in the case of P. ovalis and bryozoans. According to the second scenario, the lophophore gradually transformed from a simple oval shape to a horseshoe shape and then to a spiral shape. The four-part ground plan of the lophophoral musculature is also present in bryozoans, which is consistent with the view that the lophophorates are monophyletic.

Myoanatomy of the phoronid Phoronis ovalis: functional and phylogenetic implications

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.