NERVOUS SYSTEM 149 



the anterior end of the body and passes backwards. Any stretch 

 receptors present in the anterior segments cannot be stimulated 

 when the animal is at rest and it needs muscular contraction to occur 

 before these organs can respond. Such muscular movements 

 happen only when the animal is already moving, so the stretch 

 receptors can only prolong a movement already started by some 

 other agency. 



Removal of tension from the sense organs is followed by a 

 cessation of peristalsis. If a worm is allowed to hang by its own 

 weight and contracts rhythmically, the rhythm can be stopped by 

 surrounding the animal with water (Gray and Lissmann, 1938). 



The tension receptors may also be concerned in the homostro- 

 phic response. An earthworm is placed so that the anterior end is 

 upon a stationary surface, but the rear end rests on a moveable 

 slate. If the slate is deflected to one side the front end of the worm 

 turns until it is parallel to the direction of the rear end. The 

 number of segments involved in the response of the anterior end 

 depends upon the number of segments displaced at the hind end 

 up to a limit of about twenty segments. The receptors for this 

 response are present along the entire length of the body and 

 transmission of the impulses takes place via the ventral nerve 

 cord, transection of which prevents the response occurring. The 

 effectors for the homostrophic response, however, are confined to 

 the anterior fifteen to twenty segments. It is suggested that this 

 reflex is essential in enabling the earthworm to maintain a stable 

 position in space. No statocysts or other type of gravity receptor 

 have been described in earthworms so the activity of stretch 

 organs and their effectors seems of great importance in informing an 

 earthworm whether it is the right way up or not (Moore, 1923a), 

 although it may not explain how Liimhriciis is aware that it is 

 proceeding head uppermost to feed, or A. longa tail uppermost 

 when casting. 



The after-discharge process noted in the tension reflex is 

 sustained, as mentioned above, for some while and there is some 

 evidence relating to this, suggesting that it may be due to the 

 action of transmitter substances lingering in the system and causing 

 prolonged firing of motor nerves. Kao (1956) has found evidence 

 within the giant fibres for prolonged sensory bombardment and 

 electrical inputs continuing for some while (see p. 163). 



