Distribution of serotonin-like immunoreactive neurones in the embryonic nervous system of lymnaeid and planorbid snails.

The distribution of 5HT-like immunoreactive (5HTLI) neurones was investigated in lymnaeid and planorbid snail embryos. The time of appearance and the pattern of development of homologous 5HTLI cells in the CNS were found to be similar in all species investigated. However, 5HTLI cells characteristic only of planorbid embryos were also found. These specific neurones which were detected outside the future CNS at stage E30 of embryogenesis and which disappeared close to hatching, were named transient serotoninergic 1 cells. A correlation is suggested between the embryogenetic development of 5HTLI neurones and the development of 5HT-dependent motor behaviour in these animals.