NERVOUS SYSTEM OF PELAGIC NEMERTEANS. 



129 



The dorsal core is much smaller than the ventral and its separa- 

 tion from the latter tends to become less distinct in the posterior 

 half of the body. Toward the posterior end of the body this core 

 becomes gradually smaller, only the fibers of the ventral core 

 being involved in the posterior nerve-commissure. 



In a few genera there is but a single fibrous core, and this arises 

 mainly from the ventral ganglion. It presumably also contains 

 fibers from the dorsal ganglion, although they are usually not 

 visibly separated from the others (Figs. 5, 8). 



The position which the lateral nerves occupy in the body varies 

 greatly according to the degree of development of lateral margins. 

 In those forms in which the body is but little flattened, the nerve- 

 cords lie near the lateral margins of the body, but in the broad, 

 flattened forms they are far removed from the lateral margins, 

 being in some cases situated more than half-way toward the 

 median line (Fig. 6). In nearly all species the nerve-cords bend 

 more or less sharply laterally immediately after their origin from 

 the brain. In most forms they lie in the parenchyma near the 

 ventral surface of the body, but in those cases where large ventral 

 branches from the intestinal diverticulum force their way beneath 

 the nerve-cord, the latter is moved dorsally until it may lie about 

 midway between dorsal and ventral surfaces (Fig. 6). 



FIG. 6. Portions of transverse sections of body of six different genera, 

 showing the position of the lateral nerve cord with respect to the body 

 walls and intestinal diverticula. A, Pelagoncmcrtcs joubini Coe; B, 

 Planoncmcrtcs hbata Coe ; C, Neuronemertcs aurantiaca Coe ; D. Cuneone- 

 mcrtcs gracilis Coe; E, Ncctoncmcrtcs pclagica Cravens and Heath; F, 

 Planktonemcrtes agassizn Woodworth. 



