IV 



VERMES— NERVOUS SYSTEM 



221 



hand, the ventral chord of the Annelida can, by the separation of its symmetrical 

 halves, assume the appearance of a ladder nervous system {e.g. in the Hermellidce, 

 Fig. 147). 



In very'many Annulata (many OKgochmta and Polychmta) the brain stands in 



/^i.. 

 .< X 



flfe>,." 



FiG. 146. — Anterior part of tlie body of Cheetogaster 

 diapliaiius, from the side, after Vejdovsky. si, Sensory 

 setae ; gz, ganglionic cells of the cup-shaped organ ; bga, 

 ladder-like ventral chord of the pharyngeal region ; disi, 

 dis2, first and second dissepiments ; pm, pharyngeal mus- 

 cles ; igai, ventral ganglion in the oesophageal segment ; 

 g, brain ; pg, pharyngeal ganglion ; md, stomach-intes- 

 tine ; rt, retractors (?) of the pharynx ; sc, oesophageal 

 commissure ; dg, dorsal vessel ; hg, ventral vessel ; ce, 

 oesophagus ; ph, pharynx. 



Fig. 147.— Nervous and nephridial 

 systems in the anterior portion of the 

 body of Sabellaria alveolata, from 

 the ventral side, after E. Meyer. The 

 nervous system is made black, si-, (Eso- 

 phageal ring ; hm, ventral chord ; t, 

 tentacles ; Jc, feeler gills ; m, mouth ; 

 &, bundles of setje ; Jib, hooked setee ; 

 p, parapodia; vn, anterior pair of ne- 

 phridia ; hn, posterior nephridia through 

 which the sexual products are dis- 

 charged. 



direct connection with the hypodermis of the head segment. It often shows more 

 or less distinct lobes, always symmetrically arranged, which look like special ganglia. 

 The brain, which originally (and also ontogenetically) belongs to the head segment, can 

 in some cases stretch into the second segment ; it can even move back into the 



