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THE POPULAR SCIENCE MONTHLY. 



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 diffi- 

 cult to trace the ultimate distribution of the nerve-fibres in these creat- 

 ures ; so that, although fibres can be followed nearly up to the pigment- 

 spots, none have been detected in immediate continuity with them. 



a - 



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 com- 

 missural 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 gus- 

 tatory 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 dif- 

 ferently developed. The lateral ganglia of the Nemertidoz are re- 

 placed by two upper ganglia, connected by lateral commissures with 



