Early neurogenesis pattern in Patella coerulea (Patellogastropoda) and itspossible phylogenetic implications

In Patella coerulea (Mollusca: Gastropoda), as shown by acetylcholinesteras e (AChE) histochemistry, the primordia of the cerebral ganglia started to d ifferentiate in the young trochophore. Approximately at the same stage, two bilaterally symmetrical neural cells, each with an axon-like fibre running parallel to the midline towards the anterior, were detected in the perspec tive foot near the posterior pole. Soon later, other ectodermal cells in th eir vicinity and running parallel to their fibres, became AChE-positive, an d then, gave origin, respectively to the primordia of the pedal ganglia, an d to the rudiments of the visceral, intestinal and pleural ganglia. As shown by previous studies, in a bivalve, Mytilus, and in a polychaete, j ust as in Patella, the initial primordia of the post-trochal nerve cords di fferentiated parallel to the fibres of two posterior pioneer neural cells, In the gastropod and in the bivalve, these cells were located in the rudime nt of the foot, which was shifted towards the oral side during ontogeny (an opedial flexure). However, in Patella this occurred at an earlier stage tha n in Mytilus; then, the primordia of the pedal ganglia started to different iate in an anterior position, closer to those of the cerebral and pleural g anglia. This may be related to the development of cerebro-pedal and pleuro- pedal connectives, which are lacking in Mytilus and other bivalves. The present findings support an initial differentiation of pre- and post-tr ochal nervous system from bilaterally symmetrical neurogenesis centres loca ted near the opposite poles of the trochophore. They may also suggest an or igin of molluscs from the same ancestor as polychaetes, and a role of the a nopedial flexure in their evolutionary radiation.