NERVOUS SYSTEM 



145 



worm progresses slowly forward (Friedlander, 1894, Beidermann, 

 1904). Chaetal movements are carried out by a special musculature. 

 A retractor muscle links the inner ends of the two chaetal pairs on 

 each side of each segment. Protraction is carried out by a tent-like 

 arrangement of muscles running from the inner ends of the chaetae 

 to the body wall. There are no special muscles to rotate the chaetae 

 on their long axes, as is necessary when direction of locomotion is 

 reversed — this is done by differential action of the protractors, 

 assisted by the pressure exerted on the chaetal pairs when the 



Fig. 46. Situations affected by Friedlander (1894) and Beidermann 



(1904). (A) animal bisected and rejoined; (B) body cut, ventral 



nerve cord intact; (C) nerve cord severed, body uncut. Peristalsis 



occurs synchronously in all three cases. 



longitudinal body wall muscles contract (I. Linn, personal commu- 

 nication). 



Three early experiments indicated that the propagation of this 

 peristaltic wave from segment to segment is due to a combination 

 of factors. If an earthworm is completely severed and then the 

 two fragments rejoined by sewing thread, a peristaltic wave starting 

 at the anterior end progresses back without interruption at the cut 

 surfaces (Fig. 46). If now another animal is cut so that only the 

 ventral nerve cord remains intact the waves of the contraction still 

 pass from anterior to posterior without hindrance, the impulses 



