The plan of the central nervous system is therefore that of the Arthropoda.
The nervous system of the medusa consists of sub-epithelial ganglion-cells, which form, in the first place, a diffuse plexus of nervous tissue, as in the polyp, but developed chiefly on the subumbral surface; and which are concentrated, in the second place, to form a definite central nervous system, never found in the polyp. In Hydromedusae the central nervous system forms two concentric nerverings at the margin of the umbrella, near the base of the velum.
In connexion with the central nervous system there are usually numerous organs of special sense.
Sedation due to the drug may be increased by the concomitant use of other central nervous system depressants.
The most dramatic difference between humans and any other species, including the great apes, is found in the central nervous system.