PhD thesis

Results and discussion
21
ganglion and form a ring around the esophagus. Additional lateral and brachial
nerves emanate from the ventral ganglion (Blochmann 1892b). The nervous
system of the lecithotrophic rhynchonelliform brachiopod larvae of Terebratalia
transversa comprises two sets of four serotonergic flask-shaped cells in the
apical organ that are connected by neurites to a larval neuropil in the apical lobe
(Chapter IV). The nervous system of the lecithotrophic craniiform brachiopod
larvae of Novocrania anomala comprises four centrally positioned serotonergic
flask-shaped cells in the apical organ connected to two ventral nerve cords that
extend ventrolaterally along the body (Chapter III). Linguliform planktotrophic
brachiopod juveniles of Lingula anatina and Glottidia sp. possess a nervous
system comprising an apical ganglion as well as dorsal and ventral lophophore
nerves (Hay-Schmidt 1992). The apical ganglion of Glottidia sp. contains
numerous serotonergic cells that are associated with two serotonergic tracts
which project into the ciliary band (Hay-Schmidt 2000). This system is probably
not homologous to the apical organs found in T. transversa and N. anomala,
since there are numerous serotonergic cells in Glottidia sp. and none of these
cells are flask-shaped.
The evolution of nervous systems has been reviewed by several authors
(Bullock and Horridge 1965b; Holland 2003; Schmidt-Rhaesa 2007b; Arendt
et al. 2008; Benito-Gutiérrez and Arendt 2009; Wanninger 2009; Harzsch and
Wanninger 2010). All eumetazoans are able to transmit information between
cells. Sponges use electric signals albeit lacking neurons (Leys et al. 1999),
cnidarians have a nerve net with electrical and chemical synapses (Anderson
and Trapido-Rosenthal 2009), and bilaterians have a nervous system that often
comprises some sort of “brain” and nerve cords or neurite bundles (Rieger et al.
2010). The last common ancestor of cnidarians and bilaterians most likely had
a nerve net which developed under the control of anteroposterior patterning
genes (Westfall 1996; Westfall and Elliott 2002; Watanabe et al. 2009). The
question whether the ancestor of Protostomia and Deuterostomia had a diffuse
nervous system or a centralized nervous system is still hotly debated and a
final statement can not yet be made (Younossi-Hartenstein et al. 1997; Arendt
and Nübler-Jung 1999; Holland 2003; Lowe et al. 2003; 2006; Telford 2007;
De Robertis 2008; Reichert 2009; Harzsch and Wanninger 2010). Recent
studies showed that larval Entoprocta and adult Mollusca show a tetraneurous
condition consisting of one pair of ventral and on pair of more dorsally positioned
lateral nerve cords. In addition, the creeping-type entoproct larva and the
polyplacophoran larvae exhibit a complex apical organ consisting of around
eight centrally positioned serotonergic flask-shaped cells which are surrounded
by several peripheral cells. (Wanninger et al. 2007; Fuchs and Wanninger 2008;