20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



As discussed below, we can hypothecate for the primitive annehd 

 the presence of three pairs of longitudinal nerve cords represented 

 now by the nuchal nerves, the podial longitudinal nerves, and the 

 ventral nerve cords. Their positions in the primitive annelid, if 

 symmetrical, would be dorsolateral, lateral, and ventrolateral respec- 

 tively. As the annelid was already segmented, we can call these nerve 

 cords the nuchal, podial, and ventral chains. 



In the primitive annelid (table 2, stage i) the primal brain, repre- 

 sented by the present fore-brain, must already have constituted the 

 dominant nerve center of the body; and, judging from the structure 

 of the brain of the polychaet and especially the eunicid, it must have 

 retained its dominance till the brain was completed. The original 

 anterior ends of the ventrolateral nerve cords or "ventral chains" 

 issuing from the primal brain are represented now by the visceral 

 oesophageal cords, not by the perioesophageal connectives. The in- 

 vaginations may all have been brought about by the animal finding it 

 useful to employ appendages within its alimentary tract and there- 

 fore invaginating them. Further, as its habits changed and the 

 appendages improved, this was effected three times over. This mode 

 of origin seems especially probable in the two pharyngeal invagina- 

 tions where we still have the armature in each case; and if it is 

 accepted for them, it can with great probability be hypothecated also 

 for the oesophageal invagination where all armature is absent.^ The 

 effects of the stomodeal invaginations upon the primal ventral chains 

 may ultimately be read in detail; but if the podial chains were also 

 involved, we cannot at present cite effects due to them. 



The oesophageal invagination (table 2, stages 2 to 4). The oesopha- 

 geal nerve loops produced by this (fig. 2) are perhaps represented 

 now only by the oesophageal cords extending from the brain to the 

 supra-oesophageal ganglion (g^) of the visceral nervous system, the 

 upper half of the ganglionlike circumoesophageal ring (n co i) and 

 the continuations rearward in the main lateral pads of the oesophagus 

 {noel, figs. 4 and 5, Heider, 1925, pp. 86-88). Unknown primal 

 "ganglia" along the last are suggested by a few plus signs. The loops 

 were closed by the advance of the foremost pair of uninvaginated 

 ganglia to the brain (fore-brain) to form mid-brain I, bringing with 

 them the ventral sides of the oesophageal loops and the ventral chains 

 (table 2, stage 3). The nerves were brought up to, and appropriated 

 by, the fore-brain, whereas the mid-brain I presided over a pair of 



1 But the idea was first suggested to the author by the characters of certain 

 trilobites, where several pairs of appendages seem to be entirely hidden within 

 the alimentary tract. 



