Abstract
BackgroundEntoprocta affinities within Lophotrochozoa remain unclear. In different studies, entoprocts are considered to be related to different groups, including Cycliophora, Bryozoa, Annelida, and Mollusca. The use of modern methods to study the neuroanatomy of Entoprocta should provide new information that may be useful for phylogenetic analysis.ResultsThe anatomy of the nervous system in the colonial Barentsia discreta was studied using immunocytochemistry and transmission electron microscopy. The ganglion gives rise to several main nerves: paired lateral, aboral, and arcuate nerves, and three pairs of tentacular cords that branch out into tentacular nerves. The serotonergic nervous system includes paired esophageal perikarya and two large peripheral perikarya, each with a complex net of neurites. Each tentacle is innervated by one abfrontal and two laterofrontal neurite bundles. Sensory cells occur regularly along the abfrontal side of each tentacle. Star-like nerve cells are scattered in the epidermis of the calyx. The stalk is innervated by paired stalk nerves.ConclusionsThe neuroanatomy of the colonial Barentsia discreta is generally similar to that of solitary entoprocts but differs in the anatomy and ultrastructure of the ganglion, the number of neurite bundles in the calyx, and the distribution of serotonin in the nerve elements. A comparison of the organization of the nervous system in the Entoprocta and Bryozoa reveals many differences in tentacle innervations, which may indicate that these groups may not be closely related. Our results can not support with any certainty the homology of nervous system elements in adult entoprocts and adult “basal mollusks”.
Highlights
General morphology Barentsia discreta is a colonial species with a branching, creeping stolon (Fig. 1a, b)
The nervous system of entoprocts consists of several pairs of large nerve cords extending from the ganglion and splitting into tentacle nerves, and several pairs of longitudinal nerves extending from the ganglion and innervating the body
Different morphology and innervation of the tentacular apparatus may indicate an independent origin of the tentacular apparatus, and the absence of a close relationship between Bryozoa and Entoprocta
Summary
Entoprocts are considered to be related to different groups, including Cycliophora, Bryozoa, Annelida, and Mollusca. In a number of molecular studies, Entoprocta and Cycliophora are considered to be a sister group to Bryozoa (=Ectoprocta) [5,6,7]. The authors of the latter report studied two solitary species and described paired oral, aboral, and lateral nerves; three pairs of tentacular nerves; as well as the nerves of the calyx, stomach, rectum, esophageal nerve ring, and atrial nerve ring. To date, this has been the only detailed investigation of the neuroanatomy of Entoprocta. Ultrastructural data on the nervous system of entoprocts is extremely fragmentary [15, 21, 24]
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