NEMERTEA. 637 



The nervous system consists of a right and left cephalic ganglion 

 connected inter se by two commissures, one thick, below the proboscis and 

 its sheath, the other, thin, above them ; they are prolonged backwards into 

 two lateral nerve-cords. Both ganglia and cords consist of a peripheral 

 layer of ganglion cells 1 and a core of fibres. The ganglia are simple 

 in Carinella, Cephalothrix, and Carinoma. In other Nemertea each ganglion 

 is divisible into a ventral lobe in direct continuity with the lateral cords, 

 and a dorsal lobe, from which a posterior or third lobe is partially separated 

 in Polia, Valencinia, and Schizonemertea. This third lobe in Hoplonemertea 

 is connected to the main dorsal lobe by nerves alone, and it lies sometimes 

 behind and sometimes in front of it, e. g. in Oerstedia, Amphiporus lacti- 

 floreus, &c. The inner or postero-median side of the third lobe is formed of 

 large cells different to those of the rest of the ganglion. They sometimes 

 inclose a ciliated space. The cephalic ganglia give off various nerves, 

 some to the apex of the head, a branch to the oesophagus, and from the 

 upper proboscidian commissure a fine median nerve extending backwards 

 in relation with the sheath of the proboscis. The lateral cords are placed 

 towards the dorsal aspect of the body in Langia (Schizonemertea], towards 

 its ventral aspect in Drepanophorus (Hoplonemertea]. The supra-anal com- 

 missure is found in Hoplonemertea. The ganglia and lateral nerves of 

 Schizonemertea are deeply tinged with haemoglobin. Dorsal and ventral 

 nerves which branch dichotomously originate metamerically from the 

 lateral nerves in Hoplonemertea. But in other Nemertea the lateral nerves 

 are contained in a nervous sheath or plexus, which envelopes the body and 

 in Carinella extends beyond the ganglia to the tip of the head, a region 

 where definite nerves are found in Schizonemertea. The nervous system 

 lies in Carinella immediately outside the muscular body-walls, in Polia 

 Valencinia, and Schizonemertea internal to the outer longitudinal muscular 

 coat, and in Hoplonemertea within the body-walls. 



The organs of special sense are small tufts of tactile hairs, sometimes 

 seen at the apex of the head ; long stiff isolated hairs on the surface 

 of the body, observed only in young specimens ; eyes sometimes absent, 

 e. g. in mud-dwelling Schizonemertea, either numerous or restricted to four 

 as in Tetrastemma, in structure either mere pigment specks or provided 

 with a lens, a cellular vitreous body, retinal rods and a pigment sheath 

 as in Drepanophorus and Ampkiporus ; and otocysts stated to occur in some 

 Hoplonemertea. The two ciliated cephalic grooves situated on the head afe 

 probably not sensory organs. These structures are absent in Cephalothrix 

 and Geonemertes chalicophora. In the Palaeonemertean Carinella annulata 

 they form two simple shallow furrows at the level of the cerebral ganglia. 



1 In Carinella and Cephalothrix the ganglion layer is confined to one, the outer side of the 

 fibrous core. 



