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WESLEY R. COE. 



just behind the head. In Neuronemertes the walls of the sper- 

 maries are not provided with the thick spiral musculature found 

 in several other pelagic forms and which serves to supplement 

 the weak muscles of the body walls in the forcible discharge of 

 the spermatozoa. A similar result is accomplished however by 

 the contraction of the highly developed dorsoventral muscles 

 which closely invest the spermaries in that genus. It therefore 

 seems reasonable to suspect that these supplementary nervous 

 tissues may function in the control of this dorsoventral muscu- 

 lature throughout the body. 



There still remains the possibility of the ganglia being corre- 

 lated with sensory stimuli rather than with motor responses, but 

 their structure is unlike that of any known receptor. It seems 

 necessary, therefore, to return to the view that the ganglia of the 

 dorsal nerve supplement the nerve cells of the lateral nerve cords 

 just as these supplement those of the brain. 



FIG. 5. Transverse section of lateral nerve cord in three different genera, 

 showing the relations of the fibrous cores to the accompanying nerve cells. 

 A, Plionemertcs plana Coe, with single fibrous core (fc) and symmetrical 

 dorsal and ventral cellular layers (del and vcl) ; B, Proarmaueria pellucida 

 Coe, with small dorsal core (fc r ) ; C, Planoncmcrtcs lobata Coe, with large 

 dorsal core. 



Lateral nerves. These are direct continuations of the fibrous 

 cores of the brain lobes and are accompanied by nerve cells 

 throughout their entire length. At the posterior end of the body 

 the cords of the two sides form a broad commissure on the dorsal 

 side of the rectum (Fig. i). In most pelagic species the fibrous 

 core arising from the dorsal brain lobe is separated by a layer of 

 ganglion cells, at least in the anterior part of the body, from that 

 arising from the ventral lobe of the brain (Fig. 5). 



