?
Organization of the lophophoral nervous system in the cyclostome bryozoans confirms the relationship of the Bryozoa and Brachiozoa
The phylogeny of bryozoans is ambiguous and apparently cannot be resolved
by molecular methods alone. Morphological data from previously unstudied species may
help establishing relationships between the main groups of the Bryozoa and may help to
clarify bryozoan phylogeny. The Cyclostomata is the least studied group of bryozoans. The
nervous system of cyclostomes has not been described utilising modern methods until this
year. The recent data, which were obtained from two different cyclostome species, revealed
some lophophoral nerves may be compared with the lophophore nervous elements in other
lophophorates. In this paper, new data on organization of the nervous system in cyclostome
Cinctipora elegans and Crisia eburnea are discussed in frame of the problem of the
lophophorates monophyly. In both cyclostome species, the cerebral ganglion gives rise to
circumoral nerve ring and outer nerve ring. The circumoral nerve ring gives rise to frontal
neurite bundles of tentacles and intertentacular neurite bundles, which extend between
tentacles at their base. Each tentacle is innervated by six (in Crisia eburnea) or four (in
Cinctipora elegans) longitudinal neurite bundles. In addition to these intraepithelial neurite
bundles, the peritoneal neurites pass along the lateral sides of each tentacle between
peritoneal lining and extracellular matrix. The outer nerve ring extends along the outer
surface of the lophophore at the base of tentacles. The outer nerve ring was previously
described only in ctenostome Amathia gracilis. The outer nerve ring may be regarded
homologous to the terntacular nerve ring in phoronids and lower brachial nerve in
brachiopods. The similarities in the innervation of the lophophore and tentacles in all
lophophorates are consistent with the view that the Bryozoa and Brachiozoa are closely
related.