The nervous system of the male Dinophilus gyrociliatus (Polychaeta, Dinophilidae): II. Electron microscopical reconstruction of nervous anatomy and effector cells.

All neuronal cells in the dwarf male of the dimorphic polychaete species Dinophilus gyrociliatus were individually identified by means of serial ultrathin sections. Altogether 68 neural cells--including 40 sensory neurons and 2 glial cells--constitute a small but complex nervous system. Fifty-three neural cells are located in three pairs of ganglia and connected by paired nerve cords. The prominent frontal ganglia, each consisting of a well-developed neuropile and surrounded by 20 or 21 neural cells, represent the animal's brain. The ventral ganglia contain only 2 neurons each. The penis ganglia--four cells each--are associated with the copulatory organ. A conspicuous circumpenial fiber mass surrounds the basal part of the penis. The effector cells--22 multiciliated epidermal cells, 34 muscle cells, and different gland cells (?)--were also reconstructed and their innervation was partly elucidated. Sensory-motor neurons were unambiguously identified. They are discussed in regard to the small body size of the animal. The male's nervous organization resembles a very simple rope ladder and may represent a reduced derivative of a nervous system in normal-sized males of monomorphic species. Similarities, however, also occur with the developing nervous system of a planktotrophic metatrochophore. The neuronal organization, with its two centers (frontal ganglia and ventral ganglia vs. penis ganglia and circumpenial fiber mass), accords well with the bipartite behavioral pattern, which is entirely devoted to locomotion and copulation, respectively.