Nervous Systems 813 



Collier^''*^ suspended 20-40 intact segments of earthworms in a saline bath 

 and found no peristalsis if the worms were carefully balanced. If, however, 

 a slight stretch (0.1-1.0 gm. on the recording lever) or a tactile stimulus was 

 applied, peristalsis resulted (Fig. 305). Tension and touch elicited peristalsis 

 only if the preparation was normally innervated. When the tactile receptors 

 are anesthetized, peristalsis can be intiated by tension. 



A tension-induced peristaltic reflex can be inhibited by strong tactile stimu- 

 lation, e.g., by touching the anterior end during a tension wave^*^" (Fig- 305). 

 Whether the blocking eff'ect of an antiperistaltic wave is due to central in- 

 hibition or to refractoriness somewhere in the reflex chain is not clear. Main- 

 tained tension sets up rhythmic contractions at a maximum frequency of one 

 every two seconds; the frequency increases to a maximum as the applied 

 weight is increased. If the weight is appHed for only 0.5 second, a rhythmic 

 response occurs, a sort of after-discharge which may last as long as 3 minutes. 

 In the leech not only tension and contact but also exteroceptive stimuli from 

 the sucker are capable of eliciting peristaltic reflexes.^'^^ The leech nerve cord 

 shows asynchronous spontaneous electrical activity when isolated, and a 



Fig. 305. Reflex rhythmic contractions in an earthworm in response to tension applied 

 at arrow and inhibited by tactile stimulation of dorsal surface at time indicated by signal 

 magnet record above response. From Collier.^"" 



rhythm corresponding to the contractile rhythm when connected to an active 

 piece of the body wall. Peristalsis can be started in an earthworm^^^ by elec- 

 trical polarization with the anterior end positive and peristalsis can be in- 

 hibited by reverse polarization. 



Peristaltic locomotion in the polychaete Arenicola is likewise controlled by 

 segmental reflexes, and stimuli at the anterior and posterior ends cause oppo- 

 site parapodial movements.^^- Motion pictures of Nereis in locomotion^'''* 

 show patterns of movement successively involving clumps of 4 to 8 parapodia. 

 iAiCcompanying the wave of parapodial beat the longitudinal muscles on the 

 same side contract while those on the opposite side relax. Thus the Nereis 

 crawls in the direction in which the wave travels over the body and not, as 

 does the earthworm, in the opposite direction. 



The earthworm peristaltic reflex is segmental, bi-segmental or tri-segmental 

 in character, with reciprocal innervation of longitudinal and circular mus- 

 cles. Circular muscles contract while longitudinal muscles relax, and the 

 diameter of the animal is reduced. Then longitudinal muscles contract, 



