1^6 WESLEY R. COE. 



Of the three nervous plexuses (middle, inner, and outer) which 

 are thus formed the two latter are of such great delicacy that they 

 can be observed only under favorable conditions of preservation, 

 and it is by no means certain that they are present in all species. 



Although the radial nerves leave the middle plexus only at in- 

 tervals, the three plexuses appear to form more or less continuous 

 networks throughout the length of the anterior chamber. In some 

 forms even the middle plexus is incompletely developed, the 

 primary proboscidial nerves being connected with each other only 

 at intervals. 



The plexuses are interrupted in the middle chamber of the 

 proboscis by the interlacing of the muscular layers, and the pri- 

 mary nerves gradually lose their identity in a multitude of smaller 

 branches, some of which innervate the stylet musculature and 

 gland-cells, while others continue backward into the wall of the 

 posterior chamber. Hence in the posterior chamber the primary 

 nerves are not usually distinguishable as separate units and there 

 is but a single plexus lying between the longitudinal muscles and 

 the glandular epithelium lining the lumen. 



The smallest number of proboscidial nerves found in any pelagic 

 species is 7, and the largest is 50, the most frequent numbers being 

 from 15 to 25. In general, the species in which the individuals 

 attain the largest size have a greater number than those in which 

 the animals are but a few millimeters in length. Odd and even 

 numbers are about evenly distributed. 



Cephalic nerves. The muscles, integument and sense organs of 

 the head are supplied with a considerable number of fine nerves 

 which arise from the anterior surfaces of both dorsal and ventral 

 brain lobes (Fig. i). But since the cephalic musculature is ex- 

 tremely weak and the sense organs rudimentary, the cephalic 

 nerves are very much smaller than in most other nemerteans. 



Gastric nerves. A pair of small nerves arise from the posterior 

 end of the ventral brain lobes to supply the anterior end of the 

 digestive tract (Fig. i). These are homologous with the eso- 

 phageal nerves of the littoral forms which have the esophagus 

 well developed, but where that part of the digestive system is 

 rudimentary, as in the pelagic species, the nerves supply the stom- 

 ach. Supplementary gastric nerves, originating from the ventral 

 commissure, have also been observed in one or more species. 



