148 



THE PHYSIOLOGY OF EARTHWORMS 



be demonstrated when the earthworm fragment is hung from an 

 isotonic lever, the animal being immersed in sahne to counteract 

 the weight of its own body. 



It is essential that the weight should be applied for a certain 

 length of time, the threshold of duration. If weights are removed 

 before this period has elapsed then no peristalsis is set up, but 

 once contractions start they may continue for as long as 3 minutes 

 in response to one stimulus. This sort of response is typical of 

 reflex action and is known as ''after-discharge". This is brought on 

 by a persistence of activity within the reflex centre even after the 

 external stimulant is removed. This is demonstrated by the 



Fig. 48. After discharge invoked by the application of various 

 weights for the same length of time (15 sec), (a) 0-5 g; (b) 1-0 g; 

 (c) 1-5 g. Weight applied between arrows (from Collier, 1939). 



observation that the after-discharge period is directly proportional 

 to the magnitude of tension applied if this is done for a constant 

 short period of time (Fig. 48). There is now some neurophysio- 

 logical evidence for this belief in the earthworm (Kao, 1956) which 

 will be discussed later. 



When the stretch stimulus is maintained continuously for long 

 periods the response in the reflex arc falls off after about 25 minutes. 

 This is probably due to adaptation, and consequent raising of the 

 threshold, of the sense organs to a given set of conditions since 

 applications of a greater weight, and hence a greater tension, will 

 renew peristalsis (Collier, 1939 a, b). Though the tension reflex 

 may be of great importance in maintaining a peristalsis throughout 

 the body it cannot initiate such a contraction. Peristalsis starts at 



