NERVOUS SYSTEM OF PELAGIC XEMERTEANS. 



135 



of nerves, so that one specimen may appear to have double the 

 number found in other specimens where the proboscis is in a dif- 

 ferent state of contraction. Furthermore in some forms the num- 

 ber of nerves in the anterior end of the proboscis may be double 

 that found farther back toward the stylet region, due to the 

 gradual disappearance of each alternate, or secondary, nerve in 

 the plexus. And, finally, the number may vary to some extent in 

 different individuals, as in Nectonemertcs mirabilis where the 

 number ranges from eighteen to twenty- four. 



bm ' ' 



cm. 



i 



Im 



n- t 



FIG. ii. Diagrams of the two types of sense organs found in Cuncone- 

 mertcs gracilis Coe. A, integumentary sense organ ; B, subcutaneous sense 

 organ; bm, basement layer of surface epithelium (i) ; cm and Im, circular 

 and longitudinal musculatures of cephalic wall ; n, nerve leading to dorsal 

 brain lobe. 



The central radial branches from the nervous plexus pass be- 

 tween the bundles of longitudinal muscles and form a second, 

 delicate intermuscular plexus beneath the inner circular muscles. 

 From this plexus fine nerve-fibers supply the epithelium of the 

 papillae as well as the inner circular muscles. 



The peripheral radial branches arising from each of the primary 

 nerves form a third, still more delicate, plexus between the longi- 

 tudinal muscles and the outer circular muscular layer. This sup- 

 plies in a similar manner the more superficial layers of the pro- 

 boscis, namely, the outer circular muscles and the endothelium. 



