Recent reports indicate that neuronal elements develop in early larval stages of some Gastropoda from the Pulmonata and Opisthobranchia prior to the appearance of any ganglia of the future adult central nervous system (CNS). The present study describes similar early neuronal elements in Crepidula fornicata. A posterior FMRFamide-like immunoreactive (LIR) cell with anteriorly projected fibers was observed in the trochophore stage. Additional FMRFamide-LIR and serotonin-LIR cells and fibers were found in the apical organ in the trochophore and early veliger stages. FMRFamide-LIR and serotonin-LIR projections to the velum and foot were also detected at this time. As the veliger developed, peripheral FMRFamide-LIR and later catecholaminergic cells were located in the foot region. Also during this stage, catecholaminergic cells and processes were observed near the mouth. In addition, this study tentatively identified the first serotonin-and FMRFamide-LIR cells and fibers within the developing ganglia of the adult CNS, which appeared in close proximity to the earlier developing elements. These observations are consistent with the hypothesis that, in addition to its presumed role in the control of larval behaviors, the larval nervous system guides the development of the adult CNS. Larvae from the class Bivalvia and other invertebrate phyla also have neuronal elements marked by the presence of FMRFamide, serotonin, and catecholamines, and, therefore, this study may provide additional insights into phylogenetic relationships of the Gastropoda with other representatives of the Mollusca and different invertebrate phyla.& b d y :

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Catecholamines have previously been implicated in several important physiological processes in molluscs, including reproduction, respiration, and feeding. Much of the previous research has relied upon high-performance liquid chromatography to identify and quantify the various catecholamines and pharmacological experiments to investigate their actions. In the present report, we expand upon these studies by using histochemical techniques to investigate the distribution of catecholamine-containing cells and fibres in the central nervous system and peripheral tissues of the sea scallop, Placopecten magellanicus. Strong catecholaminergic staining was present in the somata and neuropil of all major central ganglia. Catecholamines were also abundantly stained in peripheral neurones and (or) fibres in several other tissues, including the labial palps, lips, intestine, gill filaments, foot, mantle, tentacles, and gonadal integument. It is concluded that catecholamines are widespread in the tissues of the scallop and could have potential neurotransmission roles in both the central nervous system and peripheral tissues of this species.

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