NERVOUS SYSTEM 



167 



who found that 0-1 M NaCl was the threshold value necessary to 

 fire salt-sensitive fibres, the initial frequency of the impulses increas- 

 ing as the strength of the stimulus increased. Sucrose and glucose do 

 not aff"ect the sense organs of the body segments, confirming the 

 observations of Prosser (1935b), but the prostomial nerves contain 

 a few fibres that respond to glycerol and sucrose, though not to 

 glucose. The prostomial nerves are also sensitive to quinine at a 

 concentration 10-^ M to 10-^ M. 



The importance of the hydrogen ion in governing the distribu- 

 tion of earthworms has been suspected for some time (Boden- 

 heimer, 1935; Hurwitz, 1910; Arrhenius, 1921; Satchell, 1956). 



Fig. 54. Graph to show the average time taken to withdraw the 

 prostomium from acid pH solutions in three species of earth- 

 worms X A. longa; o L. terrestris; • L. rubellus (from Laverack, 



1961a). 



DiflFerent earthworm species have slightly diflrerent habitats in 

 nature, and this may be reflected by diflFerential sensitivity to 

 acidity. By applying buflFer solutions to the body wall of these 

 species A. longa was shown to have a threshold response at pH 

 4-5, L. terrestris at pH i-2-A-l, and L. rubellus at pH 3-8 (Laverack, 

 1961a) (Fig. 54, 55). 



Motor Nerves 



Very little is known of the functioning of the motor nerves. 



M 



