The numbers of nerve cells of various sizes have been estimated in the different ganglia and lobes of the brain of Octopus vulgaris. There are about 500 million nerve cells altogether, of which 300 million are in the ganglia within the arms. The main motor contres of the brainare composed almost entirely of large neurons. Cells of intermediate size are found in the basal lobes of the supracosophagoal ganglia. The smallest neurons are minute amacrine cells, abundant in parts of the optic and in the subfrontal and vertical lobes, perhaps having inhibitory actions.