Nervous System. 537 



agus with a nervous ring. From this, bands of nerve fibres pass back- 

 ward along the ventral side of the bod} r , connecting the cephalic gan- 

 glion with each of the segments, by which the action of all may be con- 

 trolled in harmony by the sight stimulus. If, while a Centipede is in 

 motion, its head be cut off, the legs will continue to move, and so carry 

 the body forward. But the guiding sense of sight being cut off with 

 the head, there is no avoidance of objects which ma} r happen to be in 

 the way. The body runs against any obstacle, but if it be lower than 

 itself it will mount over it and go on, but if it is too high, the body 

 stops while the legs, not controlled by a guiding sense, will continue to 

 move as if ever} r thing were all right. This motion is caused by the ' 

 stimulus of the contact of the feet with the ground, and is the so-called 

 reflex action. This action can be started in the limbs, when the animal 

 is cut in two or the head cut off, by irritating the cut end of the 

 nerves. If a middle section of the ventral nerve chain be removed 

 while the Centipede is alive, the legs opposite that part become motion- 

 less, but those in front move in co-ordination with the head ganglion, 

 while those back of the mutilation move by the touch stimulus, but not 

 in harmony with those in front. 



The Praj'ing Mantis, an insect of the grasshopper kind, is another 

 animal in which the effect of reflex stimuli can be readity observed. It 

 has six legs, but the front pair are differentiated into a sort of claws, 

 which are usually held aloft in an attitude which suggests its name, but 

 is really an attitude of preying instead of praying, for thus it lies in 

 wait. If the head of this animal be cut off it will still stand holding 

 up its arms, and if any object be placed between them they will close 

 upon it as when alive. If the segment to which these arms are at- 

 tached, be cut off, the insect will continue to stand and resist attempts 

 to overthrow it, and the wings will perform the same agitated movements 

 as when the head is present. Moreover, if any object be placed within 

 the arms attached to the abscinded segment, they will still' grasp it as 

 before. l These facts prove, as pointed out by Carpenter, that the or- 

 dinary movements of the insect are prompted by the reflex action of 

 the ganglia of the ventral cord, such action being the continuance 

 through the ganglia of the direct stimuli of touch. In life, the general 

 movement of the body is directed by the stimuli through the eye, while 

 the desire or impulse to move at all is internal, and comes from the 

 state of the stomach, nerves from this quarter mingling their stimuli 

 with those of sense in the optic ganglion, thus making this a real brain, 

 albeit a very simple one. 



The Gyrinus Dytiscus is a water beetle which slides about on the 

 surface of summer streams. If one of these be beheaded and placed 



1 See Carpenter's Mental Physiology. 



