INVOLUNTARY RHYTHMICAL MOVEMENTS. 109 



reflex action. The stimulus is convej^ed to the abdo- 

 minal ganglia through afferent nerves, and is reflected 

 from them, by efferent nerves, to the abdominal muscles. 



But this is not all. Under these circumstances it will 

 be seen that the abdominal limbs all swing backwards 

 and forwards, simultaneously, with an even stroke ; while 

 the vent opens and shuts with a regular rhythm. Of 

 course, these movements imply correspondingly regular 

 alternate contractions and relaxations of certain sets of 

 muscles ; and these, again, imply regularly recrn'ring 

 efferent impulses from the abdominal ganglia. The fact 

 that these impulses proceed from the abdominal ganglia, 

 may be shown in two ways : first, by destrojdng these 

 ganglia in one somite after another, when the move- 

 ments in each somite at once permanently cease ; and, 

 secondly, by irritating the surface of the abdomen, when 

 the movements are temporarily inhibited by the stimula- 

 tion of the afferent nerves. Whether these movements are 

 properly reflex, that is, arise from incessant new afferent 

 impulses of unknown origin, or whether they depend on the 

 periodical accumulation and discharge of nervous energy in 

 the ganglia themselves, or upon periodical exhaustion and 

 restoration of the irritability of the muscles, is unknown. 

 It is sufficient for the present purpose to use the facts as 

 evidence of the peculiar co-ordinative function of ganglia. 



The crayfish, as we have seen, avoids light; and the 

 slightest touch of one of its antennae gives rise to active 

 motions of the whole body. In fact, the animal's posi* 



