Composition of neurons, their structure and neuromediatory specialization in the Japanese scallop ganglia have been studied by means of morphological, morphometrical and histochemical methods. Three groups of neurons, differing in their histophysiological characteristics have been defined. Large neurons are cholinergic ones. A well developed system of smooth membranes and large amount of cytosomes are specific for them. Middle size and small neurons of the I type contain a granular endoplasmic reticulum, elementary neurosecretory peptide granules and are considered as peptidergic. Small neurons of the II type possess increased nuclear-plasmic relations, moderately developed endoplasmic reticulum, positively react to biogenic amines and according to their mediatory specialization are monoaminergic.