Spiralia is a vast clade of Metazoa comprising large and well-known organisms, e.g., Annelida
and Mollusca, but also many microscopic animals such as Gastrotricha or Gnathifera (including,
Rotifera) of the often overlooked meiofauna. To date, the phylogeny and morphology of
Spiralia have been difficult to resolve and understand. The present thesis focuses on spiralian
meiofauna to i) reconstruct the phylogeny of this clade using transcriptomics and place
enigmatic meiofaunal taxa and ii) resolve the morphology of three important taxa, mainly
employing confocal laser scanning microscopy and immunohistochemistry: the spiralian
incertae sedis, Lobatocerebrum, the recently described monospecific phylum Micrognathozoa
(Gnathifera), and an early branching Gastrotricha, Diuronotus aspetos.
The new spiralian phylogeny reveals with high support that the deepest branches of Spiralia
consist of meiofaunal representatives, that Gnathifera is the sister group of remaining Spiralia,
that Gastrotricha+Platyhelminthes branches off next and that Lobatocerebrum is an Annelida.
The morphological surveys of the musculature, nervous system, glands, and ciliation on three
phylogenetically distinct taxa yield more insight into their evolution: Lobatocerebrum is an
aberrant annelid showing only few common traits with Annelida, yet, our detailed studies
unravel putative resemblances of muscular, nervous and glandular system to previous findings
in annelids. Micrognathozoa shows more resemblances with Rotifera than Gnathostomulida
(these three taxa together forming Gnathifera). Furthermore, we could infer possible
plesiomorphic states of Gnathifera such as the paired ventro-lateral nerve chords (shared with
many Spiralia) as well as recover putative Gnathifera apomorphies such as the pharyngeal
ganglion; all adding new information on the evolution of this group. Diuronotus aspetos shows
a unique combination of muscular traits not easily traceable, but in contrast the nervous
system traits can be compared in high details, hereby bridging to other Chaetonotida
(Gastrotricha). Moreover, we describe new gastrotrich characters such as the ciliary pattern or
a system of pharyngeal canals of possible importance for future comparative approaches.
These different studies show that information on rare and phylogenetically isolated animals
with their unique combination of neural and muscular characters are necessary to understand
the evolution of Spiralia. Also, several organ systems should be considered for systematic
comparisons, here emphasized with ciliary and glandular systems in Micrognathozoa,
Gastrotricha and Lobatocerebrum showing potential phylogenetic information.