of the sheath of the proboscis. Each ganglion is pyriform in shape, and connected with its fellow by means of two commissures, one of which passes over, and the other underneath, the proboscis. It is difficult to trace the ultimate distribution of the nerve-fibres in these creatures; so that, although fibres can be followed nearly up to the pigment-spots, none have been detected in immediate continuity with them.
Fig. 1.—Head and Brain of Nemertes.
The inferior commissure between the two ganglionic masses is shorter and broader than .the upper, and, while it serves in part to bring the two ganglia into communication, it is also partly composed of commissural fibres, uniting the two great lateral nerve-trunks. These start from the ganglia, and, proceeding along the sides of the body, give off numerous branches to the longitudinal and circular muscles between which they are situated.
The pyriform ganglia are mostly of a pink or reddish color, and they are crowded with small nerve-cells. They represent the brain as it exists in these animals, and we have here, perhaps, a type of the simplest form which this organ could assume among active creatures possessing a distinct bilateral symmetry. Tactile and possibly gustatory impressions, together with impressions produced by light or darkness, doubtless come from the anterior extremity of the organism to the pyriform ganglia on either side, and are thence reflected along correlated channels in the great efferent bundles, proceeding to the muscles on one or both sides of the body, and also to the muscular proboscis. Other departments of the nervous system may exist in these animals, though as yet none have been detected.
In the common earthworm the nervous system is somewhat differently developed. The lateral ganglia of the Nemertidæ are replaced by two upper ganglia, connected by lateral commissures with
