Catecholaminergic neurons in the nervous system of Neorhabdocoela (Turbellaria)

Mapping of catecholaminergic (CAergic) neurons in the nervous system has been performed in 5 species of turbellarians of the order Neorhabdocoela: in three species of typhloplanides, Mesostoma lingua, Bothromesostoma essenii, and Rhynchomesostoma rostratum, and in two species of Dalyellioida, Castrella truncata and Gieysztoria cuspidata. In spite of an essential diversity of their orthogones, the number of cerebral neurons varies insignificantly: from 4 to 5 pairs at 3 variants of geometry of their arrangement. In the presence of the medial paddle-shaped outgrowth of neuropil, two pairs of anterior cerebral neurons are located on its both sides, while 2 pairs of posterior neurons, in the inferior lateral regions of neuropil. The absence of the medial outgrowth and a stretching of neuropil provided a fan-shaped arrangement of cerebral neurons behind the eyes and on the sides of the pharynx. The three-store arrangement of cerebral neurons is revealed in the presence of a cone-shaped trunk at the anterior end of the body, when all 5 pairs of neurons are connected to each other with longitudinal and transverse processes. The rosette-shaped pharynx is innervated with five pharyngeal nerves, while the barrel-shaped pharynx, with six nerves, each of them with one bipolar neuron. From 2 to 5 pairs of neurons of the L-group were revealed. Homology of the L-group neurons has been confirmed in the order Neorhabdocoela. The total number of CAergic neurons varies from 24 to 29.