COE : NEMERTEANS OF WEST AND NORTHWEST COASTS. 



49 



60). In Cephalothrix they are situated in the midst of the 

 longitudinal muscles throughout the whole length of the body. 



In the Heteroneniertea the nerve cords lie immediately outside 

 the circular muscles and beneath a greatly thickened outer longi- 

 tudinal muscular layer, while in the Hoplonemertea they occupy 

 positions quite internal to the musculature of the body walls and 

 are surrounded by the body parenchyma (Text-figs. 16, 17). 



In nearly all nemerteans the lateral nerves are situated well 

 toward the lateral margins of the body, although in Drepano- 



..ifK 



.J)e 



Fig. 17. —Zijffonemerfef: thalassina. Transverse section of body immediately back 

 of brain, showing intestinal caeca {ic) reaching forward to abut against dorsal 

 ganglia. Ocelli (oc) extend back of brain, and occupy positions, as shown, 

 immediately above and external to lateral nerve. 



PHORUS they lie comparatively near together toward the ventral 

 surface of the body. 



In general histological structure the brain and lateral nerves are 

 closely similar, although each has nerve cells of a somewhat differ- 

 ent character. Each has a central core of very delicate fibrils of 

 connective tissue in which slender nerve fibers are imbedded. The 

 central core is surrounded by a delicate sheath of connective tissue,^ 

 outside of which the massive layer of nerve cells is situated. An 

 outer sheath of connective tissue covers this peripheral layer of 

 nerve cells. 



