PhD thesis - Biologisk Institut - Københavns Universitet

25 ResultsSerotonergic and FMRFamidergic nervous system development in sipunculanworms (chapters I – III)In the planktotrophic species Phascolosoma agassizii, the first detectable serotonergicand FMRFamidergic nervous structures are visible in the anterior region of thetrochophore larvae. A pair of neurites emerging from the apical organ contributes tothe ventral nerve cords. During further development, interconnecting commissuresand pairs of perikarya (four serotonergic and one pair of FMRFamidergic perikarya)appear subsequently along the ventral nerve cords from anterior to posterior, resultingin a rope-ladder like ventral nervous system. Due to migration of serotonergicperikarya and fusion of serotonergic ventral nerve cords this metameric arrangementis lost towards metamorphosis, and gives rise to the single non-metameric adultventral nervous system (chapters II and III).In the lecithotrophic species Themiste pyroides and Thysanocardia nigra,neurogenesis is similar to Phascolosoma agassizii (Figs. 4-6). The first serotonergicand FMRFamidergic cells are in the apical organ and a pair of neurites contributes tothe ventral nerve cords (Figs. 4A, 6A). In contrast to P. agassizii, a first, second, and athird pair of serotonergic perikarya appear already in the trochophore larvae of T.pyroides and T. nigra in an anterior to posterior progression, while later the fourthpair appears in the pelagosphera larva (Figs. 4A-D, 5E, H).