PhD thesis

20 Results and discussion
Myogenesis
Results of larval myogenesis and adult myoanatomy are presented in Chapters
II and III.
Actin and myosin are molecules present in all metazoans including basal groups
such as sponges and Trichoplax (Thiemann and Ruthmann 1989; Kanzawa et
al. 1995). It has been proposed that the basal pattern of musculature in the
bilaterian ancestor was a grid of outer circular and inner longitudinal musculature,
the Hautmuskelschlauch (HMS), which has in some taxa been modified in
combination with the evolution of hard exoskeletons (Schmidt-Rhaesa 2007a).
Brachiopods have discrete bundles of muscle fibers that control the movement
of the valves and the tentacles. Brachiopods have further myoepithelia which
are found on the inner side of coelomic epithelia, in the parietal bands, in mantle
lobes, and in the lophophore (Williams et al. 1997). Additionally, I could show
that adults of the species Joania cordata, Argyrotheca cistellula, Novocrania
anomala, and Terebratalia transversa contain discrete bundles of mantle
retractor muscles (Chapters II, III), a character that is probably present in all
brachiopods.
The larval musculature is similar among the rhynchonelliform brachiopods
investigated herein (Chapter II). Remnants of a HMS could not be distinguished.
Accordingly, if the ancestor of Brachiopoda had a HMS, it was lost during the
evolution of this phylum. Interestingly, the larval musculature of the craniiform
brachiopod Novocrania anomala is very different from the musculature of
the investigated rhynchonelliform brachiopod larvae (Chapter III). This hints
towards an early split in the evolution of these two groups. This is confirmed by
the fossil record, which estimates the split between the rhynchonelliform and
craniiform clade to have taken place before the Ordovician 485 million years
ago (Freeman and Lundelius 2005).
Neurogenesis with special focus on the apical organ of
lophotrochozoan larvae
Results on neurogenesis in brachiopod larvae and juveniles are presented in
Chapters III and IV.
Adult rhynchonelliform brachiopods have a nervous system which is concentrated
around the esophagus and comprises two ganglia, one dorsal and one ventral of
the esophagus, as well as circumenteric nerves that innervate the lophophore,
ventral mantle nerves, and dorsal mantle nerves (van Bemmelen 1883; Bullock
and Horridge 1965a). The nervous system of adult Novocrania anomala lacks
the dorsal ganglion. The circumenteric nerves emanate laterally from the ventral