The nervous system in the cyclostome bryozoan Crisia eburnea as revealed by transmission electron and confocal laser scanning microscopy
Introduction: Among bryozoans, cyclostome anatomy is the least studied by modern methods. New data on the
nervous system fill the gap in our knowledge and make morphological analysis much more fruitful to resolve some
questions of bryozoan evolution and phylogeny.
Results: The nervous system of cyclostome Crisia eburnea was studied by transmission electron microscopy and
confocal laser scanning microscopy. The cerebral ganglion has an upper concavity and a small inner cavity filled
with cilia and microvilli, thus exhibiting features of neuroepithelium. The cerebral ganglion is associated with the
circumoral nerve ring, the circumpharyngeal nerve ring, and the outer nerve ring. Each tentacle has six longitudinal
neurite bundles. The body wall is innervated by thick paired longitudinal nerves. Circular nerves are associated with
atrial sphincter. A membranous sac, cardia, and caecum all have nervous plexus.
Conclusion: The nervous system of the cyclostome C. eburnea combines phylactolaemate and gymnolaemate
features. Innervation of tentacles by six neurite bundles is similar of that in Phylactolaemata. The presence of
circumpharyngeal nerve ring and outer nerve ring is characteristic of both, Cyclostomata and Gymnolaemata. The
structure of the cerebral ganglion may be regarded as a result of transformation of hypothetical ancestral
neuroepithelium. Primitive cerebral ganglion and combination of nerve plexus and cords in the nervous system of
C. eburnea allows to suggest that the nerve system topography of C. eburnea may represent an ancestral state of
nervous system organization in Bryozoa. Several scenarios describing evolution of the cerebral ganglion in different
bryozoan groups are proposed.