NO. 8 THK I'OLYCIIAKT KAW IJ 



JJcidcr, after satisfyinj^ himself as t*; tlie nervous character of the 

 bodies, suggests that they are motor centers for tlic muscles of the 

 jaw pads; and though he could not trace ctmnections, he suggests 

 that their posterior horns may connect with the nerves from the 

 terminal visceral oesfj[;hagwd ganglion ()■.,, and that th(Mr anterior 

 ventral horns connect with the tw(j visceral pharyngeal cords. 



We can agree with ileider that the function (>{ this pair oi nerve 

 centers is connected with the muscles of the jaw pads; hut we can 

 go further and suggest, on the theory here advanced, that eixch X- 

 form nerve center is due to coalescence of the ganglia (lateral at 

 that time; of tlie segments, the apjjendages of wliich arc represented 

 hy the upper jaw apjjaratus, and the highly modified muscles of 

 which are represented hy the muscles of the jaw .sacs. There is close 

 coordination of the appendages, anrl there is union of the ganglia 

 serving them. 



The two pharyngeal cords of the visceral nervous .system arise 

 from the sides of the fore-brain via tlie anterior roots of the oesopha- 

 geal commissures, and extenrl rearward to the side wall of the pharynx. 

 They are traced by 1 1 eider as far back as opposite the .second ventral 

 ganglion and podial nerves. 'Jheir course is along the deepest 

 (farthest sideways) part of the main sac, ventrad of the lateral bases 

 of the jaw pads, and closely clinging to the hypodermis. 



If Heider's supposed connections are correct we have here a nerve 

 course from the fore-brain rearward to the main visceral nerve 

 center in the pharynx, forming one side only of a visceral loop, the 

 other side of which is to be found only in the oesophageal section. 

 If at one time the ancestor of Eunice had separate oesophageal and 

 pharyngeal nerve loops, their adjoining halves forming an opposite 

 loop have di.sappeared. But if connection between the outside halves 

 was effected, the connection of the inside halves with the brain would 

 be redundant. Such a connection might be caused by the .sharp 

 folding of the hypodermis which the stomodeum exhibits, or might 

 arise when the lip between the two invaginations became bifid. 



On the general theory applied above to the oesophageal invagina- 

 tion, the evolution of the upper jaw sac was followed by the advance 

 of the foremost unaffected segmental pair of ganglia to join the brain, 

 thus completing the previously open pharyngeal loop. This is repre- 

 sented by the middle pair of the three mid-brain "ganglia," numbered 

 II (the second in size and the second in antiquity) of the additions 

 to the brain. It appears to have arrived like its predecessor behind 

 the fore-brain, and to have pushed the previous addition to the rear; 

 for the anterior section of the "mid-brain," considered below, was 



