[The golden age of comparative morphology: laser scanning microscopy and neurogenesis of trochophore animals].

Immunochemical labeling of neuronal elements and laser confocal microscopy have considerably expanded the capacity of comparative morphology and allowed us to monitor the neurogenesis of various trochophore animals at the level of individual identified neurons and their projections. It has been demonstrated that many generally accepted concepts of the larval nervous system and the phylogenetic theories constructed on this basis are incorrect. Comparative analysis has demonstrated that the orthogonal brain is absent at all developmental stages in the representative Lophotrochozoa members. Fundamental differences in the structure and development of the nervous system have been found in the trochophores belonging to different taxonomic groups within Lophotrochozoa; these differences demonstrate that the trochophore larva in these groups are not homologous, while their similarity is most likely a result of convergence. Our results challenge the concept of trochophore as the ancestral form common for all trochophore animals. It is necessary to exclude from phylogenetic discussions the orthogon as a basic plan for the structure of the nervous system and the trochophore as an ancestral form for all Lophtrochozoa.