THE NEMERTINI 



185 



O.E 



Gl 



lobe ; hence the organ receives the name " cerebral organ." In its 

 simplest form, in the Protonemertini, it is a mere groove in the epi- 

 dermis not extending deeper than the basement membrane ; it is 

 lined by ciliated cells, and at the bottom are large gland cells ; 

 the organ is supplied by nerves from the brain. In Carinella 

 rabicunda and others the groove becomes an oblique canal, the 

 blind end of which is surrounded by a mass of ganglion cells, lying 

 outside the cutis. In the higher forms the canal penetrates deeper 

 into the body as far as the brain (Fig. XXIII. ). The gland cells and 

 the nerve tissue associate with it, increase in amount, and the canal 

 becomes differentiated into two regions an extra - ganglionic 

 " lateral canal," and an intra-ganglionic " cerebral canal " (C), which 

 frequently terminates in an en- 

 larged sac. In Drepanopliorus the 

 cerebral canal is quite exceptional, 

 in that it bifurcates one branch 

 terminating in a sac with sensory 

 epithelium (Fig. XXIV. (7), the 

 other being glandular (GT) ; 

 this, in D. crassus, extends back- c 

 wards beyond the brain as a free 

 tube. In several genera of this 

 order the cerebral organ lies in 

 front of the brain, e.g. Tetra- 

 stemma, sp. of Eunemertes and of 

 Ampliiporus ; in others it lies at 

 the side ; and in still others, be- 

 hind the brain in which case it 

 attains a great size. In all cases 

 the organ is separate from the^ 

 brain, from which it receives 

 nerves. 



The lateral canal of the cere- 

 bral organ opens to the exterior 

 in relation to a furrow or groove 



FIG. XXIV. 



Cerebral organ of Drepanophoruscrassus (from 

 place Perrier, after Burger). Schematic horizontal 



class. ' 



section ; A, hinder end of dorsal ganglion ; B, 

 ProtO- nerve to cerebral organ ; C, cerebral sac 

 the dilated outer limb of the original canal 

 embedded in a mass of nerve tissue ; E, 



on the head, which is somewhat 

 variously disposed in tl 

 It is simplest in the 

 nemertini, being a shallow, 

 vertical furrow the " cephalic 

 furrow," marking the head 



.__ < ~ 7 . Utl'VUUa \J\JL U1U1I Ul UllU Ul^ctll , L/JJ/J oj-n--* U^AV, VM. 



from the trunk (Fig. XXV.), and lateral canal of the organ; P, pigment; R, 



in most of these the cerebral L 



organ is little else than a deeper part of the furrow T , but in C. ruU- 

 cunda opens independently of the latter. In Metanemertini and 

 in the genus Eupolia this furrow becomes crescentic with its con- 



