THE EARTHWORM AND ANNELIDS IN GENERAL 177 



terior end of the body in somite III is a bi-lobed mass of nervous 

 tissue (Fig. 90, 2); this is called the brain, or supra-pharyngeal 

 ganglion, because of its position on the dorsal surface of the 

 pharynx. Two large nerve cords, the circum-pharyngeal connec- 

 tives (j), pass around the pharynx one on either side, connecting 

 the brain with a pair of ganglia lying beneath the pharynx in 



FIG. 90. Diagram of the anterior end of an earthworm to show the arrange- 

 ment of the nervous system. /, prostomium ; 2, brain ; j, circum- 

 pharyngeal connective; 4, sub-pharyngeal ganglion; 5, mouth; 6, 

 pharynx ; 7, setae ; 8, tactile nerves to prostomium ; p, dorsal nerves ; 

 10, ventral nerves. (From Shipley and MacBride.) 



somite IV. These are the sub-pharyngeal ganglia (4). Every 

 somite posterior to IV contains a ganglionic mass which is con- 

 nected by a nerve cord with the ganglion in the preceding somite, 

 and also, with the exception of the last, with that in the succeed- 

 ing somite. This connected row of ganglia is called the ventral 

 nerve cord, and, together with the supra-pharyngeal ganglia, con- 

 stitutes the central nervous system. The nerves which pass from 

 the central nervous system to all parts of the body, and which 

 pass to it from the body wall and internal organs, constitute the 



, N 



