INVOLUNTARY RHYTHMICAI, MOVEMENTS. 109 
reflex action. The stimulus is conveyed 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 recurring 
efferent impulses from the abdominal ganglia. The fact 
that these impulses proceed from the abdominal ganglia, 
may be shown in two ways: first, by destroying 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 antenne gives rise to active 
motions of the whole body. In fact, the animal’s posi- 
