The golden age of comparative morphology: Laser scanning microscopy and neurogenesis in trochophore animals

Immunochemical labelling of neuronal elements and laser confocal microscopy have considerably expanded the capacity of comparative morphology, and allowed us to study neurogenesis in various trochophore animals at the level of identified neurons. The data obtained have impeached some popular phylogenetic theories. Thus, the comparative study has shown that in the representative Trochozoan species, the orthogonal brain is absent at all developmental stages. Fundamental differences in neurogenesis and neuroarchitecture between trochophores from different taxa suggest that trochophore-like larvae are not homologous, and their likeness is most probably the result of convergence. Our data support the intercalation hypothesis of the origin of indirect development with a trochophore-like larva. It seems reasonable to exclude from phylogenetic discussions the orthogon and the trochophore as an ancestral form common for Lophotrochozoa.