NERVOUS SYSTEM OK PELAGIC NEMERTEANS. 



127 



divided into two or more lobes (Figs. 3, 4). The nerve itself 

 lies external to the circular muscular layer, while the ganglia are 

 internal to these muscles and interposed between the bundles of 

 longitudinal muscles. The broad connection between ganglion 

 and nerve therefore requires the penetration of the thin circular 

 musculature. This is accomplished by the mere separation of ad- 

 jacent circular fibers so as to leave a narrow slit through which 

 the connection passes. 



^-mmim 



FIG. 4. Transverse sections of six of the ganglia of the dorsal nerve 

 of Neuroncmcrtes aurantiaca Coa, showing variations in shape and their 

 relations to the nerve core. A, Dorsal nerve between two adjacent ganglia. 

 B, Small ganglion closely fused with dorsal nerve. C, Large ganglion 

 with ventral group of nerve cells. D, Surface view of ganglion, its con- 

 nection with dorsal nerve being in an adjacent section. E, Large ganglion 

 with three lobes of nerve cells and distinct intermuscular plexus. F ' , Large 

 ganglion with four lobes of nerve cells ;imp, intermuscular plexus; other 

 lettering as in Fig. 2. 



The cells composing the ganglia are similar in size and appear- 

 ance to those which accompany the lateral nerve cords, and the 

 cell boundaries are likewise rarely to be distinguished (Fig. 4). 

 It has been difficult to determine the precise nature of these organs. 

 That their component cells are the source of fibers which inner- 

 vate the integument and musculatures seems highly probable. 

 And it is also presumable that they supplement the nerve cells 

 found in all species accompanying the lateral nerve cords, but 

 there is no evidence that these latter cells are less abundant in 

 this species than in other pelagic forms. A suggestion that may 

 have some degree of plausibility is that they are associated with 

 the great development of the dorsoventral musculature, particu- 

 larly in the region where the spermaries are situated in the male, 



