Anatomt/ and Histology of Nemertines. 40o 



possess only one pan- of ncurochord-cclls, wliicli belong to 

 the brain, and only a single ])air of ujibranchcd neurochords, 

 which run through the brain and the lateral eords. 



The entire mass of the central nervous system is enveloped 

 in a neurilemma. The fibrillar central substance of the 

 lateral cords in all cases, and throughout Group II. that of 

 the brain as well, is also enclosed in an inner neurilemma 

 and sharply marked off from the coat of ganglion-cells. 



The perij)heral nervous system is represented by nerves 

 and nerve-sheaths. Nerves supply the cephalic extremity, 

 the eyes, and the lateral pits. A pair of nerves, which arise 

 from the ventral ganglion, runs back to the oesophagus ; a 

 ])rccisely analogous pair, springing from the ventral com- 

 missure, sup])lics the ])roboscis in Groups I. and II. In 

 (jroup HI. the proboscis is innervated by means of numerous 

 stems, arising from the brain. In some species of the first 

 group, and in all those of the third, the lateral organs are 

 united by nerves to the dorsal ganglia. In all the groups 

 the lateral cords give off nerves, which are arranged meta- 

 merically in Groups II. and III. In Carinina the nerve- 

 sheath assumes an epithelial position, in accordance witii the 

 situation of the lateral cords ; in the other genera of this 

 group the sheath is subepithelial. In Group II. the nerve- 

 sheath is generally situated outside the circular muscle-layer, 

 but it may occur within it, as in Langia and Cerehratulus. 

 In Group III. nerve-sheaths are not found. The nerve- 

 sheaths are characterized by the presence of a median dorsal 

 nerve, which runs through them in the longitudinal axis of 

 the body. This nerve also persists in Group III., only in 

 this case it maintains an intermuscular position, above the 

 circular muscle-layer. A second and smaller nerve of this 

 kind, lying within the circular muscle-layer, is characteristic 

 of the first two groups only. 



In close connexion with the nervous system come the 

 sense-organs — the subepithelial eyes (the pigment-cups of 

 which are directed outwards), the lateral organs, the accessory 

 lateral grooves lined with columnar epithelium {Drepaao- 

 phorus), and the terminal cephalic grooves {Cerehratulus). 



The lateral organs arc placed in the same position as the 

 brain, and in a portion of the genera belonging to the first 

 group and in all those of the second and third they fuse with 

 the dorsal ganglion, behind which they always lie. In the 

 Enopla they occupy an independent position, being connected 

 with the upper ganglion by nerves only, and generally lying 

 to the side of it, though they may occupy a position in front 

 of it, towards the cephalic extremity. As special formations 



