NEMERTEA. 



267 



above and below the proboscis sheath and each divided into two 

 lobes, a dorsal and ventral. The ventral lobes are continued back- 

 wards as the lateral nerve-cords (Fig. 221), which in some, cases 

 unite posteriorly above the intestine by a supra -anal commissure. 

 The dorsal lobe (Fig. 221, O) is partly or completely divided into an 

 anterior and posterior lobe. In the armed forms (Fig. 221, L) this 

 third lobe is com- 

 pletely separated off 

 and united with the 

 anterior only by 

 nerves. This pos- 

 terior part of the 

 dorsal lobe contains 

 in nearly all forms 

 an epithelium - lined 

 cavity which com- 

 municates with the 

 exterior by a fine 

 ciliated tube. The 

 latter opens, in the 

 forms with head-slits, 

 into the slits, in the 

 other forms on the 

 side of the head. 

 The whole apparatus 

 including the brain 

 lobe constitutes the 

 lateral organs, or as 

 it is sometimes called, 

 the cerebral organs. 

 The central nervous 

 system lies in Cari- 

 nella outside the 

 muscular layers ; in 

 the Heteronemertini 

 between the external 



longitudinal and the circular layer; and in the Metanemertini com- 

 pletely within the muscles. In all, except the Metanemertini^ in 

 which the central nervous system gives off nerve-cords, there is a 

 complete nerve-sheath of a reticulate nervous substance, occupying 

 the same position with regard to the muscles as do the central 



FIG. 221. Diagram of the nervous system of Drepanophorus 

 Lankesteri (from Perrier after Hubrecht). C dorsal lobes of 

 the cerebral ganglia ; t nerves of the proboscis ; s sensory 

 nerves of head ; ce stomatogastric nerves ; L posterior part 

 of the dorsal lobes of the brain in which are the lateral 

 organs ; opening of lateral organ ; n peripheral nerves ; 

 T lateral nerve-cords ; v, k transverse commissures. 



