The role of transient larval FMRFa-ergic neurons of the freshwater snails, Helisoma trivolvis and Lymnaea stagnalis, in osmoregulation and neurodifferentiation was investigated. It was shown that: (i) FMRFa and an FMRFa antibody do not reach their targets when injected into the egg capsule; (ii) long-term incubations of the embryos in neomycin and hyperosmotic solutions of sea water, NaCl and sucrose during the definite developmental stages lead to a special malformation-hydropia; (iii) hydropia coincides with an extensive larval FMRFa-ergic nervous system; (iv) the increased level of FMRFa causes earlier serotonin synthesis by the neurons of the visceral loop ganglia.